Update python environment

[elchupanebrej]

• Drop python 3.6 because of EOL
• Drop pytest<5 (Actually there wasn't reason to support 4.x version because they are about 3.4/2.7 python support, which already dropped)
• Fix source distribution build

[codecov]

Codecov Report

Merging #495 (b05dbe0) into master (2fb5a11) will decrease coverage by 0.45%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #495      +/-   ##
==========================================
- Coverage   95.48%   95.03%   -0.46%     
==========================================
  Files          48       48              
  Lines        1573     1570       -3     
  Branches      171      166       -5     
==========================================
- Hits         1502     1492      -10     
- Misses         44       48       +4     
- Partials       27       30       +3     

Impacted Files	Coverage Δ
tests/scripts/test_generate.py	100.00% <ø> (ø)
tests/utils.py	66.66% <0.00%> (-33.34%)	⬇️
tests/conftest.py	77.77% <0.00%> (-22.23%)	⬇️
tests/feature/test_tags.py	96.15% <0.00%> (-3.85%)	⬇️
pytest_bdd/hooks.py	100.00% <0.00%> (ø)

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[youtux]

Thanks @elchupanebrej!