Keyrings.cryptfile

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Kudos to Kyle Altendorf for this release!

Thank you for finally finding and fixing a nasty issue, that I introduced
by careless syncing with keyrings.alt. See:
https://github.com/frispete/keyrings.cryptfile/commit/ae871c09aec4bb84cab7e756bf25b89c6eb04394#r90506129

- Switch back to a single character assoc joiner '\0' and add backwards
compatibility support for the intervening versions 1.3.6 through 1.3.8.
Fixes: https://github.com/frispete/keyrings.cryptfile/issues/15
- Add tests for this change and setup a github CI pipeline.
- Switch to jaraco.classes.properties.

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- Apply rename of _escape.py in convert.py as well. Thanks, Justin.
- Fix TestEncryptedFileKeyring.test_wrong_password()
- Relocate tests to keyrings/cryptfile/

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- Attempt to fix explicit keyring_key handling

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- Merge current versions of file.py, file_base.py with tests from keyrings.alt
- Adopt cryptfile tests to pytest
- Rename _escape.py to escape.py

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- Adjust to current keyring testing

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- Remove support for Python 2.7 and keyring versions prior to 19.0.0