add Python 3.9 support

[noviluni]

It will be released the 05/09/2020, just before the next planned release of dateparser.

[noviluni]

should I change the used distro (xenial)? it is, in fact, necessary? 🤔

[Gallaecio]

should I change the used distro (xenial)? it is, in fact, necessary?

I would remove the distro from all entries as long as test pass. In Scrapy we have a few jobs where it is needed, but in Dateparser it may not be needed at all.

[codecov]

Codecov Report

Merging #732 into master will increase coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #732      +/-   ##
==========================================
+ Coverage   95.23%   95.34%   +0.11%     
==========================================
  Files         302      304       +2     
  Lines        2560     2622      +62     
==========================================
+ Hits         2438     2500      +62     
  Misses        122      122              

Impacted Files	Coverage Δ
dateparser/__init__.py	100.00% <0.00%> (ø)
dateparser/conf.py	100.00% <0.00%> (ø)

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