Add linter

[lepture]

Added flake8 and mypy. There are many issues found my mypy.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (460e5eb) 99.32% compared to head (e1be85d) 99.32%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main       #3   +/-   ##
=======================================
  Coverage   99.32%   99.32%           
=======================================
  Files          46       46           
  Lines        2379     2379           
  Branches      354      354           
=======================================
  Hits         2363     2363           
  Misses          8        8           
  Partials        8        8           

Flag	Coverage Δ
unittests	99.32% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/joserfc/jws.py	100.00% <ø> (ø)
src/joserfc/rfc7516/types.py	100.00% <ø> (ø)
src/joserfc/rfc7517/models.py	100.00% <100.00%> (ø)

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[lepture]

@Viicos Can you help me with the mypy configuration.

[Viicos]

Would you like mypy to suceed first (by turning off some switches), so that we can gradually improve typing?

[lepture]

@Viicos sure. If you can join the discord sever, we can discuss about the typing on discord.

[lepture]

I'll merge the code at first. Need to create a wip-mypy branch.