🛠 Added typings for public api (and mypy checks)

[k4black]

Typings for all publicly available functions have been added. upd: missed it: Related to #215

Motivation

When integrating python-jose into the fastapi (PR #3305) library, there were additional difficulties due to the lack of type annotation in this library.

Changes

Changes were made only in the typings and did not affect the 'logic'. One exception:

• Switching from Mapping to dict when checking the payload type.
  For a full discussion, see a similar question in PyJWT lib (Issue #392).

And small fixes in confusing examples and removed typings in docstrings (bytes instead of str, etc)

upd: added py.typed file according to PEP-561

Full typings 😳

I was there, Gandalf. I was there 3000 years ago...

It's really hard. Really. You can check out the typings branch in my fork repository.
But it's worth doing

1. a lot of errors in the annotation in the comments to the function
2. lots of confusing names and re-uses typed variables in the backends code
3. Augean Stables (typing-version) in the file cryptography_backend.py

[codecov[bot]]

Codecov Report

Merging #265 (3496708) into master (cfbb868) will increase coverage by 0.08%. The diff coverage is 98.73%.

@@            Coverage Diff             @@
##           master     #265      +/-   ##
==========================================
+ Coverage   92.94%   93.03%   +0.08%     
==========================================
  Files          15       15              
  Lines        1418     1450      +32     
==========================================
+ Hits         1318     1349      +31     
- Misses        100      101       +1     

Impacted Files	Coverage Δ
jose/jws.py	94.48% <94.11%> (-0.56%)	:arrow_down:
jose/backends/__init__.py	100.00% <100.00%> (ø)
jose/constants.py	100.00% <100.00%> (ø)
jose/jwe.py	81.15% <100.00%> (ø)
jose/jwk.py	82.45% <100.00%> (+0.31%)	:arrow_up:
jose/jwt.py	98.27% <100.00%> (+0.26%)	:arrow_up:
jose/utils.py	100.00% <100.00%> (ø)

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[k4black]

Please, see this PR - I moved pytest.ini config to pyproject.toml.
Maybe this will come in handy

[k4black]

@mpdavis or @blag, can you review this PR, please?
Is it relevant?