Remove Python 2.7 and 3.5 support, add Python 3.9 support

[Gallaecio]

Once merged, we can also merge #167

To do:

• Comment for skipping W503
• Review usage of tests/py3-ignores.txt
• Stop using the u prefix in literal strings
• Remove encoding declarations

[codecov]

Codecov Report

Merging #168 (0b05957) into master (7843f6f) will decrease coverage by 1.04%.
The diff coverage is 94.54%.

❗ Current head 0b05957 differs from pull request most recent head 0579cad. Consider uploading reports for the commit 0579cad to get more accurate results

@@            Coverage Diff             @@
##           master     #168      +/-   ##
==========================================
- Coverage   95.45%   94.40%   -1.05%     
==========================================
  Files           7        7              
  Lines         484      465      -19     
  Branches       99       92       -7     
==========================================
- Hits          462      439      -23     
- Misses         15       16       +1     
- Partials        7       10       +3     

Impacted Files	Coverage Δ
w3lib/encoding.py	98.52% <0.00%> (-1.48%)	⬇️
w3lib/util.py	60.00% <60.00%> (-9.70%)	⬇️
w3lib/form.py	96.00% <100.00%> (-0.43%)	⬇️
w3lib/html.py	93.69% <100.00%> (-0.17%)	⬇️
w3lib/url.py	97.96% <100.00%> (-0.09%)	⬇️
w3lib/http.py	96.87% <0.00%> (-3.13%)	⬇️

[elacuesta]

Nice 💪

[noviluni]

Is the tests/py3-ignores.txt file used?

It was created in 2014. I see there was a similar file in Scrapy (scrapy/scrapy@350aa67), but it was used in the conftest. Where is this file used in this project? Could we remove it on this PR or do you prefer to open a new PR?