v1.3.3: Allow Groupby Index

[jmcarpenter2]

Resolves bug I noticed about passing in df.index to by argument of groupby

[codecov-commenter]

Codecov Report

Merging #195 (41ca1b7) into master (99ed0df) will increase coverage by 0.36%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master     #195      +/-   ##
==========================================
+ Coverage   94.89%   95.26%   +0.36%     
==========================================
  Files           6        6              
  Lines        1137     1141       +4     
==========================================
+ Hits         1079     1087       +8     
+ Misses         58       54       -4     

Impacted Files	Coverage Δ
swifter/swifter.py	87.06% <66.66%> (+1.17%)	⬆️
swifter/__init__.py	90.00% <100.00%> (ø)
swifter/swifter_tests.py	98.70% <100.00%> (+0.01%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 99ed0df...41ca1b7. Read the comment docs.