Support old 'toml' package for Focal

[cottsay]

I'm not thrilled about using the deprecated 'toml' package, but tomli isn't available in Focal as a system package. The changes here will only fall back to 'toml' as a last resort, which should allow us to keep Focal support alive for a while longer.

This should address the deb regression on Focal in colcon-core 0.18.0.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.32%. Comparing base (cb9bd9d) to head (de9a48f).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
colcon_core/python_project/spec.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #670      +/-   ##
==========================================
- Coverage   87.36%   87.32%   -0.05%     
==========================================
  Files          68       68              
  Lines        3942     3945       +3     
  Branches      777      777              
==========================================
+ Hits         3444     3445       +1     
- Misses        394      396       +2     
  Partials      104      104              

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