Fix PyPi failures

[jonn-smith]

My fork of Pomegranate is preventing release to PyPi properly:

…
Checking dist/maslongbow-0.5.39-py2.py3-none-any.whl: PASSED with warnings
WARNING  `long_description_content_type` missing. defaulting to `text/x-rst`.   
Checking dist/maslongbow-0.5.39.tar.gz: PASSED with warnings
WARNING  `long_description_content_type` missing. defaulting to `text/x-rst`.   
Uploading distributions to https://upload.pypi.org/legacy/
Uploading maslongbow-0.5.39-py2.py3-none-any.whl
[25](https://github.com/broadinstitute/longbow/runs/8102287644?check_suite_focus=true#step:13:26)l
  0% ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 0.0/110.2 kB • --:-- • ?
100% ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 110.2/110.2 kB • 00:00 • 6.3 MB/s
100% ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 110.2/110.2 kB • 00:00 • 6.3 MB/s
25hWARNING  Error during upload. Retry with the --verbose option for more details. 
ERROR    HTTPError: 400 Bad Request from https://upload.pypi.org/legacy/        
         Invalid value for requires_dist. Error: Can't have direct dependency:  
         'pomegranate @                                                         
         git+https://github.com/jonn-smith/pomegranate.git@d80ec9[30](https://github.com/broadinstitute/longbow/runs/8102287644?check_suite_focus=true#step:13:31)4bba4c42ee394
         3c887754fd78885cd55'

This needs to be fixed by either moving to a proper version of pomegranate (assuming the PR gets merged in to fix the heap overflow), by releasing my fork and using that, or by removing pomegranate and using something else.