Fix bug w/ url_utils.filepath_from_url + rel paths with a single "/"

[ssteinbach]

• Fix traceback when using relative paths with one "/" and not two "/" were triggering a traceback - ie "./blah" but not "./blah/blah" in url_utils.
• url_utils is used by the otioz adapter (Which is where this was originally reported)

Fixes #1817

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.72%. Comparing base (c0e97b0) to head (8bd1e99).
Report is 57 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1862      +/-   ##
==========================================
+ Coverage   84.11%   84.72%   +0.61%     
==========================================
  Files         198      177      -21     
  Lines       22241    12809    -9432     
  Branches     4687     1191    -3496     
==========================================
- Hits        18709    10853    -7856     
+ Misses       2610     1773     -837     
+ Partials      922      183     -739     

Flag	Coverage Δ
py-unittests	84.72% <100.00%> (+0.61%)	⬆️

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

Files with missing lines	Coverage Δ
src/py-opentimelineio/opentimelineio/url_utils.py	85.18% <100.00%> (ø)
tests/test_url_conversions.py	95.45% <100.00%> (+0.10%)	⬆️

... and 129 files with indirect coverage changes

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[apetrynet]

Looks good to me