Introduce type annotations

[jugmac00]

You can run the type checker with tox -e mypy.

This is just a start.

• A lot of type annotations are still missing.
• The type checker is not yet using the strict option.
• Type annotations are not yet enforced in CI.

modified: tnefparse/cmdline.py modified: tnefparse/codepage.py modified: tnefparse/tnef.py modified: tox.ini

P.S.: As there is quite some development going on, and as typing the complete project will take quite some time (at least for me), I suggest to merge this soon, as otherwise a lot of merge conflicts could arise. Python offers the concept of gradual typing, so, why no use it?

[codecov[bot]]

Codecov Report

Merging #91 (fc3e0bc) into master (f5b7b17) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #91   +/-   ##
=======================================
  Coverage   93.33%   93.33%           
=======================================
  Files           7        7           
  Lines        1290     1291    +1     
  Branches      117      117           
=======================================
+ Hits         1204     1205    +1     
  Misses         64       64           
  Partials       22       22           

Impacted Files	Coverage Δ
tnefparse/codepage.py	80.95% <100.00%> (+0.95%)	:arrow_up:
tnefparse/tnef.py	92.06% <100.00%> (ø)

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