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 90.90909% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 83.30%. Comparing base (c7d8194) to head (fe92c5f).

Files	Patch %	Lines
colcon_core/shell/template/__init__.py	90.90%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #627      +/-   ##
==========================================
- Coverage   83.37%   83.30%   -0.08%     
==========================================
  Files          66       66              
  Lines        3789     3791       +2     
  Branches      737      737              
==========================================
- Hits         3159     3158       -1     
- Misses        556      557       +1     
- Partials       74       76       +2     

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