Update python versions

[kasium]

• Remove support for python 3.5/3.6
• Add CI checks for python 3.8/3.9
• Add pypi keywords for python 3.7-3.9

Part of #319

[codecov]

Codecov Report

Merging #325 (8703c08) into master (abf7a25) will increase coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #325      +/-   ##
==========================================
+ Coverage   95.43%   95.55%   +0.11%     
==========================================
  Files          25       25              
  Lines        1973     2026      +53     
  Branches      274      274              
==========================================
+ Hits         1883     1936      +53     
  Misses         56       56              
  Partials       34       34              

Impacted Files	Coverage Δ
jaeger_client/rate_limiter.py	100.00% <0.00%> (ø)
jaeger_client/span_context.py	100.00% <0.00%> (ø)
jaeger_client/span.py	96.66% <0.00%> (+0.11%)	⬆️
jaeger_client/reporter.py	93.20% <0.00%> (+0.17%)	⬆️
crossdock/server/endtoend.py	96.66% <0.00%> (+0.23%)	⬆️
jaeger_client/config.py	96.19% <0.00%> (+0.53%)	⬆️
crossdock/server/server.py	88.40% <0.00%> (+0.80%)	⬆️

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[kasium]

@yurishkuro any clue why the other checks don't run?