Apply Flake8 on source code

[jugmac00]

... and enforce it via CI, ie Travis and GitHub Actions.

Some notes This is a huge diff, which is usually no good idea, but almost inevitable for a first time linting.

There is a very thorough test suite for this package - otherwise I would not have done such a massive linting.

Some notable finding I did not change this bare except statement https://github.com/koodaamo/tnefparse/blob/491ec5b4504b9e7479e9152fa4b641fd27387c60/tnefparse/util.py#L46-L52 as in Python 2 one could expect a ValueError, whereas in Python 3 there can be OverFlowError or OsError - let's fix that when Python 2.7 gets dropped. Also, the except branch is not covered. https://docs.python.org/3/library/datetime.html#datetime.datetime.utcfromtimestamp

I did not touch the unused import statements in the package's __init__.py file, as this may be seen as a public API https://github.com/koodaamo/tnefparse/blob/491ec5b4504b9e7479e9152fa4b641fd27387c60/tnefparse/__init__.py#L3

The many # noqa comments are not excactly beautiful, but... there is not much of a choice https://github.com/koodaamo/tnefparse/blob/491ec5b4504b9e7479e9152fa4b641fd27387c60/tnefparse/mapi.py#L28-L44

The drop in coverage comes from splitting uncovered lines into two, e.g.

attr_data = data[offset : offset + 16]; offset += 16

This fixes #66

[codecov[bot]]

Codecov Report

Merging #67 into master will decrease coverage by 0.00%. The diff coverage is 92.06%.

@@            Coverage Diff             @@
##           master      #67      +/-   ##
==========================================
- Coverage   92.98%   92.98%   -0.01%     
==========================================
  Files           7        7              
  Lines        1297     1311      +14     
  Branches      121      121              
==========================================
+ Hits         1206     1219      +13     
- Misses         69       70       +1     
  Partials       22       22              

Impacted Files	Coverage Δ
tnefparse/properties.py	100.00% <ø> (ø)
tnefparse/util.py	72.22% <40.00%> (ø)
tnefparse/mapi.py	80.35% <96.00%> (+1.13%)	:arrow_up:
tnefparse/__init__.py	71.42% <100.00%> (ø)
tnefparse/codepage.py	80.00% <100.00%> (ø)
tnefparse/tnef.py	92.06% <100.00%> (ø)

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