Oops! Transport lines effectviely never appear aside from raw bridge
descriptors (which we never see), so I didn't have a live example to
test with.

Now we have one. DocTor's descriptor validation check is failing with...

  03/05/2020 00:35:33 [WARNING] Unable to retrieve the extrainfo descriptors: Transport line has a malformed address: transport obfs4 [2001:985:e77:5:fd34:f56b:c2d1:e98c]:10394 cert=dJ/a+vnP+eFv7FDaVUqWCVlyrqf8FlOva2YAEkDUwiGQuorZf4Oc6FXSdyn8b4pUmZj/WA,iat-mode=0

Caught thanks to GeKo.

Removing a private class that supported version rule chains (for example,
"check if I'm greater than 1.2.2 in the 1.2 series, or 1.3.5 in the 1.3
series"). This was only used for a single requirement check which has now
been removed.

We can always revive this class in the future if it's useful.

I doubt anyone has ever used event versions from our Requirement constants.
Retaining these within the event classes but dropping the clutter from our
version module.

Audited our library's cryptography imports for exception handling when
unavailable, and consistently document when it's required.

Python includes sqlite3 as a builtin, but when you compile python yourself it
isn't included by default. Our stem.manual attempted to account for this but
our tests didn't, and with python3 the module raises a ModuleNotFoundError
rather than an ImportError.

  Traceback (most recent call last):
    File "/home/atagar/Desktop/stem/test/task.py", line 160, in _import_tests
      importlib.import_module(module.rsplit('.', 1)[0])
    File "/home/atagar/Python-3.8.1/Lib/importlib/__init__.py", line 127, in import_module
      return _bootstrap._gcd_import(name[level:], package, level)
    File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
    File "<frozen importlib._bootstrap>", line 991, in _find_and_load
    File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
    File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
    File "<frozen importlib._bootstrap_external>", line 783, in exec_module
    File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
    File "/home/atagar/Desktop/stem/test/unit/manual.py", line 8, in <module>
      import sqlite3
    File "/home/atagar/Python-3.8.1/Lib/sqlite3/__init__.py", line 23, in <module>
      from sqlite3.dbapi2 import *
    File "/home/atagar/Python-3.8.1/Lib/sqlite3/dbapi2.py", line 27, in <module>
      from _sqlite3 import *
  ModuleNotFoundError: No module named '_sqlite3'

When an Ed25519Certificate was created through from_base64() we appended an
undocumented 'encoded' attribute with the base64 content we were created from.
Our to_base64() method computes the exact same thing.

This attribute was only used in a single place for server descriptor
validation.

If 'encoded' and to_base64() mismatches that is a bug. Fiddling with these I
cound't come up with a scenario where that is a case, so dropping the redundant
attribute.

Also merging _validate_server_desc_signing_key() into its sole caller.

Our prereq module provides dependency checks, mostly for python versioning
(now moot) and the cryptography module.

It is not only cleaner but less error prone to perform dependency checks down
where their imports are performed rather than upfront. This way import changes
can't fall out of sync with our upfront checks.

Oops, when removing unused 'sign' arguments I accidently dropped our
skip_crypto_validation argument here. George and I invested quite a bit of
effort into creating cryptographically valid HSv3 descriptors, but HSv2
descriptors aren't.

This caused the following test failures...

  ======================================================================
  ERROR: test_minimal_hidden_service_descriptor
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "/home/atagar/Desktop/stem/test/unit/descriptor/hidden_service_v2.py", line 414, in test_minimal_hidden_service_descriptor
      desc = HiddenServiceDescriptorV2.create()
    File "/home/atagar/Desktop/stem/stem/descriptor/hidden_service.py", line 702, in create
      return cls(cls.content(attr, exclude), validate = validate)
    File "/home/atagar/Desktop/stem/stem/descriptor/hidden_service.py", line 723, in __init__
      signed_digest = self._digest_for_signature(self.permanent_key, self.signature)
    File "/home/atagar/Desktop/stem/stem/descriptor/__init__.py", line 1043, in _digest_for_signature
      key = load_der_public_key(_bytes_for_block(signing_key), default_backend())
    File "/home/atagar/.local/lib/python3.7/site-packages/cryptography/hazmat/primitives/serialization/base.py", line 32, in load_der_public_key
      return backend.load_der_public_key(data)
    File "/home/atagar/.local/lib/python3.7/site-packages/cryptography/hazmat/backends/openssl/backend.py", line 1182, in load_der_public_key
      self._handle_key_loading_error()
    File "/home/atagar/.local/lib/python3.7/site-packages/cryptography/hazmat/backends/openssl/backend.py", line 1373, in _handle_key_loading_error
      raise ValueError("Could not deserialize key data.")
  ValueError: Could not deserialize key data.

Oops, I forgot to install cryptography under python 3.7 so I didn't exercise
several tests. This doesn't fix all the failures but corrects the most obvious
(we removed the sign argument when it was irrelevant).

Oops, I removed our LogBuffer in commit 76b6b49 but it's still used by
run_tests.py when we have a '--log' argument. Caught thanks to our Jenkins
tests...

  Traceback (most recent call last):
    File "./run_tests.py", line 478, in <module>
      main()
    File "./run_tests.py", line 238, in main
      handler = logging_buffer = stem.util.log.LogBuffer(args.logging_runlevel)
  AttributeError: module 'stem.util.log' has no attribute 'LogBuffer'