Add support for Python 3.9

[m1so]

No description provided.

[cosmicBboy]

thanks for the contribution @m1so! updated this PR since 3.9 is only currently available on the conda-forge channel

[cosmicBboy]

there are some issues building a conda environment on python 3.9 right now conda/conda#10267 I think deps need to be packaged for 3.9 before this can work on current CI setup.

[codecov-io]

Codecov Report

Merging #307 (c3833f1) into master (5a3ed31) will increase coverage by 0.23%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #307      +/-   ##
==========================================
+ Coverage   98.41%   98.64%   +0.23%     
==========================================
  Files          18       18              
  Lines        1707     1702       -5     
==========================================
- Hits         1680     1679       -1     
+ Misses         27       23       -4     

Impacted Files	Coverage Δ
pandera/io.py	97.82% <0.00%> (ø)
pandera/model.py	100.00% <0.00%> (ø)
pandera/checks.py	98.42% <0.00%> (ø)
pandera/errors.py	100.00% <0.00%> (ø)
pandera/typing.py	100.00% <0.00%> (ø)
pandera/decorators.py	100.00% <0.00%> (ø)
pandera/hypotheses.py	92.72% <0.00%> (ø)
pandera/error_formatters.py	100.00% <0.00%> (ø)
pandera/model_components.py	100.00% <0.00%> (ø)
pandera/schema_statistics.py	100.00% <0.00%> (ø)
... and 3 more

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[cosmicBboy]

@m1so had to make a few more changes to diff, but now tests pass. Thanks for getting the ball rolling on this PR!