Add support for editable installs

[JeanChristopheMorinPerso]

Link the Issue(s) this Pull Request is related to.

Fixes #1286.

Summarize your change.

seetuptools added back editable installs not too long ago and it now supports extensions!

This PR adds support for installing OTIO in editable mode, using pip install -e ..

Note that due to the nature of OTIO, the binaries are not editable. There is not much we can do in regard to that since every time the C++ code changed, we need to re-compile the extension. But at least the Python files are editable.

[codecov-commenter]

Codecov Report

Merging #1460 (ad62152) into main (d59a358) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1460   +/-   ##
=======================================
  Coverage   78.89%   78.89%           
=======================================
  Files         200      200           
  Lines       21724    21724           
  Branches     4403     4403           
=======================================
  Hits        17140    17140           
  Misses       2354     2354           
  Partials     2230     2230           

Flag	Coverage Δ
py-unittests	78.89% <ø> (ø)

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[meshula]

This change, overall looks good to me, and seems like it will make the iteration process slightly less difficult to teach.

A general comment ~ you've left some general comments here in the PR of explanation for details that one can't know without special knowledge. All the comments, in my opinion, belong in the source as comments for reference by future maintainers.

[meshula]

Thanks :)