Use Pyproject2Conda to manage build envs

[mdekstrand]

This switches our build environment management from conda-lock to pyproject2conda, with pre-rendered environment files saved in the envs dir. Closes #340.

[codeclimate]

Code Climate has analyzed commit 8c873b2 and detected 14 issues on this pull request.

Here's the issue category breakdown:

Category	Count
Style	14

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[codecov]

Codecov Report

Attention: 32 lines in your changes are missing coverage. Please review.

Comparison is base (f2205fc) 91.08% compared to head (8c873b2) 89.98%.
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #345      +/-   ##
==========================================
- Coverage   91.08%   89.98%   -1.11%     
==========================================
  Files          39       41       +2     
  Lines        3187     3234      +47     
==========================================
+ Hits         2903     2910       +7     
- Misses        284      324      +40     

Files	Coverage Δ
lenskit/datasets/__init__.py	100.00% <100.00%> (ø)
lenskit/datasets/movielens.py	69.23% <100.00%> (ø)
lenskit/util/__init__.py	87.14% <100.00%> (+2.14%)	⬆️
lenskit/datasets/fetch.py	27.27% <27.27%> (ø)

... and 2 files with indirect coverage changes

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