DOC: Docstring and README improvements

[scottclowe]

• General docstring improvements.
• Fix rendering of markup in compiled documentation (verified locally against the HTML docs). I had to add :term: around some of the type entries which weren't getting picked up automatically. There are still some str entries which did not get picked up and still wouldn't work with a :term: attribute added.
• Fix some intersphinx issues (numpy moved their documentation, which broke that intersphinx link up). However, PIL.Image is still not working correctly; I was unable to resolve that.
• A couple of fixes to the name of the variable in the docstring, where it didn't match the name of the variable accepted by the function/method.
• Blacken sample code within docstrings.
• Change copyright year to 2021.
• Add Python 3.9 to our list of Trove classifiers, since we now test on python3.9 in our CI.

[codecov]

Codecov Report

Merging #175 (1aac123) into master (73cee3b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #175   +/-   ##
=======================================
  Coverage   93.03%   93.03%           
=======================================
  Files           8        8           
  Lines         818      818           
  Branches      161      161           
=======================================
  Hits          761      761           
  Misses         29       29           
  Partials       28       28           

Flag	Coverage Δ
unittests	92.90% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
fissa/ROI.py	78.26% <ø> (ø)
fissa/core.py	96.22% <ø> (ø)
fissa/deltaf.py	100.00% <ø> (ø)
fissa/extraction.py	100.00% <ø> (ø)
fissa/neuropil.py	91.42% <ø> (ø)
fissa/readimagejrois.py	91.38% <ø> (ø)
fissa/roitools.py	88.98% <ø> (ø)

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