Cryptidy

This is a maintenance release that mainly adds nested trackeback removal, in order to not diluate the essential error messages.
Doing so made me add another python 2.7 compatibility hack. Keeping Python2.7+ compatibility becomes harder and harder btw, but as long as `pycryptodomex` will keep working, I will too.

This release replaces v1.2.0 which has been removed, because of a typo in error handling that made failed decryption process abort without proper message.

Features
- Added PEP-0409 statements in order to avoid nested tracebacks

Misc
- Refactor header & footer handling
- Removed python 3.5 and 3.6 from the test matrix (github doesn't provide them anymore)
- Shifted linter from python 3.9 to 3.10
- Removed cryptidy < 1.0.0 compatibility layer

This release adds two wrapper functions for both symmetric and asymmetric encryption protocols:
- encrypt_message_hf() and decrypt_message_hf()

These funtions allow to add a header/footer for encrypted message identification (useful when someone gives you an encrypted string and you need to know what it represents, just like RSA key headers and footers).
They also allow setting random header and footers.

Other changes include:
- Added an error message when trying to unpickle a class which is not imported yet
- Added pypy 3.8 to test matrix

Bugfixes
- Fixed importing typing library on python 2.7
- Fixed a problem when encoded timestamp was missing

Misc
- Added newer python versions to testing matrix
- Added black linter, moved all linting to Python 3.9 only

This release adresses two bugs:
- Timestamps were not stored as UTC so different timezones could end up making the decryption process assume timestamps were in future
- Future or too old timestamps should not prevent decryption, only warn about possible problems

This also improves the general github CI on both Windows and Linux platforms.

This is the first public release of cryptidy, which was tested and used for a couple of years in a bigger project.
Released as module since testing is easier and patches are welcome ;)