Python3.8fix multiprocessing work around

[MLKaufman]

Thank you to the authors for their work on this tool.

I found a workaround that allows the multiprocessing as written to work on Python versions later than 3.7.
By setting:
mp.set_start_method('fork')
we can override the new default on some systems to use the spawn method

See these for references on the issue:
deeptools/HiCExplorer#628
https://chrissardegna.com/blog/multiprocessing-changes-python-3-8/

Since Python 3.7 is now officially depreciated, it is necessary to support more recent versions of python.

I have not tested this modification on versions newer than 3.8, but I would expect it to work.

I only bumped the required python version to a max of 3.8 but we can modify that when we confirm that additional versions are working.

This should fix this posted issue:
#137