Replace Travis-CI with GitHub actions

[rafalp]

Fixes #268 and #483

Removes Travis CI and adds GH actions instead.

Note: pip-compile insists on including typed-ast which is not supported for Python 3.8 and outright crashes installation on Python 3.9, hence the separate dev deps file for python 3.9.

Maybe we should drop Mypy/pylint/black from dev deps and install it only on CI step?

[codecov]

Codecov Report

Merging #484 (b2a1f94) into master (d435262) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #484      +/-   ##
==========================================
- Coverage   98.18%   98.16%   -0.02%     
==========================================
  Files         104      104              
  Lines        5123     5125       +2     
==========================================
+ Hits         5030     5031       +1     
- Misses         93       94       +1     

Impacted Files	Coverage Δ
ariadne/graphql.py	89.92% <ø> (ø)
ariadne/asgi.py	95.21% <100.00%> (-0.03%)	⬇️
ariadne/contrib/django/views.py	100.00% <100.00%> (ø)
ariadne/contrib/federation/utils.py	100.00% <100.00%> (ø)
ariadne/extensions.py	100.00% <100.00%> (ø)
ariadne/schema_visitor.py	94.16% <100.00%> (ø)
ariadne/types.py	96.55% <100.00%> (+0.12%)	⬆️
ariadne/utils.py	100.00% <100.00%> (ø)
ariadne/wsgi.py	95.65% <100.00%> (-0.04%)	⬇️
tests/test_camel_case_to_snake_case_convertion.py	100.00% <100.00%> (ø)
... and 1 more

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[bogdal]

LGTM 👍