WIP: Compatibility with EmPy 4

[cottsay]

I believe this is the minimum set of changes necessary to use EmPy 4.

This needs to be updated to be compatible with EmPy 3 as well before being merged - there is no way we can cold-cut to v4.

[codecov]

Codecov Report

Attention: Patch coverage is 48.38710% with 16 lines in your changes missing coverage. Please review.

Project coverage is 87.01%. Comparing base (f9072e8) to head (a470f49).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
colcon_core/shell/template/__init__.py	48.38%	10 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #627      +/-   ##
==========================================
- Coverage   87.34%   87.01%   -0.34%     
==========================================
  Files          67       67              
  Lines        3913     3934      +21     
  Branches      776      782       +6     
==========================================
+ Hits         3418     3423       +5     
- Misses        391      401      +10     
- Partials      104      110       +6     

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[clalancette]

FYI; I've rebased this onto the latest, and also merged the Empy 3/4 compatibility work from @ahcorde into this. I don't think this is quite ready to go in, but I think it is closer now.