Remove redundant code and fix pylint

[hugovk]

EOL Python 2.7 and 3.5 were dropped in #168 so we can remove some redundant code and upgrade
upgrade Python syntax with https://github.com/asottile/pyupgrade --py36-plus.

Also add python_requires to help pip and fix some deprecation warnings.

[codecov]

Codecov Report

Merging #175 (bf2c90e) into master (2d76c99) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #175      +/-   ##
==========================================
- Coverage   95.68%   95.68%   -0.01%     
==========================================
  Files           7        7              
  Lines         464      463       -1     
  Branches       88       88              
==========================================
- Hits          444      443       -1     
  Misses         10       10              
  Partials       10       10              

Impacted Files	Coverage Δ
w3lib/encoding.py	98.66% <100.00%> (-0.02%)	⬇️
w3lib/html.py	95.45% <100.00%> (ø)
w3lib/http.py	100.00% <100.00%> (ø)
w3lib/url.py	98.02% <100.00%> (ø)