Fix code coverage detection

[ttung]

Exclude the test files from the calculations of what code is covered.

Also exclude the file that's autogenerated by versioneer.

This is based on #1111

[codecov-io]

Codecov Report

Merging #1115 into tonytung-mv-tests will decrease coverage by 16.54%.
The diff coverage is n/a.

@@                  Coverage Diff                   @@
##           tonytung-mv-tests    #1115       +/-   ##
======================================================
- Coverage              88.87%   72.32%   -16.55%     
======================================================
  Files                    167      167               
  Lines                   6516     6516               
======================================================
- Hits                    5791     4713     -1078     
- Misses                   725     1803     +1078

Impacted Files	Coverage Δ
starfish/imagestack/test/test_imagestack_index.py	13.79% <0%> (-86.21%)	⬇️
starfish/intensity_table/test/test_concatenate.py	15.38% <0%> (-84.62%)	⬇️
starfish/codebook/test/test_from_code_array.py	20.28% <0%> (-79.72%)	⬇️
starfish/config/test/test_config.py	21.89% <0%> (-78.11%)	⬇️
starfish/imagestack/test/test_imagestack.py	20.8% <0%> (-77.19%)	⬇️
starfish/image/_filter/test/test_white_tophat.py	23.33% <0%> (-76.67%)	⬇️
...tarfish/codebook/test/test_per_round_max_decode.py	25.58% <0%> (-74.42%)	⬇️
starfish/imagestack/test/test_apply.py	26.08% <0%> (-73.92%)	⬇️
...rfish/intensity_table/test/test_from_imagestack.py	28.57% <0%> (-71.43%)	⬇️
..._decoder/test/test_intensities_to_decoded_image.py	28.57% <0%> (-71.43%)	⬇️
... and 35 more

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