change pyo3 as optional dependency in native Python Parser

[sehz]

@zsol great work on Python Parser. This makes it more accessible outside this project.

Summary

Native Rust parser has pyo3 as a mandatory dependency. Since I am only interested in Python Parser, I want to change to optional dependency to reduce compile and build complexity. With the change,pyo3 is still included as a default feature thus everything is the same as before.

Pyo3 related code changes (conversion) are segregated together.

Test Plan

The existing test plan should be sufficient since this is feature separation.

[zsol]

Thanks for the PR! Looks largely good, but I'd like to change two things before merging:

1. let's run something in CI to make sure the crate isn't broken without the new feature (at least it should build)
2. can we call the new feature py or python? Just in case later we decide to change pyo3 to something else

[sehz]

Thanks for the PR! Looks largely good, but I'd like to change two things before merging:

1. let's run something in CI to make sure the crate isn't broken without the new feature (at least it should build)
2. can we call the new feature py or python? Just in case later we decide to change pyo3 to something else

Done. Change to py feature flag

[codecov-commenter]

Codecov Report

Merging #598 (a339051) into main (31ba5bf) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #598      +/-   ##
==========================================
+ Coverage   94.73%   94.76%   +0.03%     
==========================================
  Files         242      245       +3     
  Lines       24843    25038     +195     
==========================================
+ Hits        23534    23728     +194     
- Misses       1309     1310       +1     

Impacted Files	Coverage Δ
libcst/tests/test_batched_visitor.py	100.00% <0.00%> (ø)
...emod/visitors/tests/test_apply_type_annotations.py	100.00% <0.00%> (ø)
libcst/tests/test_add_slots.py	100.00% <0.00%> (ø)
...demod/commands/tests/test_convert_type_comments.py	91.30% <0.00%> (ø)
libcst/codemod/commands/convert_type_comments.py	94.00% <0.00%> (ø)
libcst/_nodes/whitespace.py	98.94% <0.00%> (+0.01%)	⬆️
libcst/_nodes/statement.py	95.15% <0.00%> (+0.01%)	⬆️
libcst/_nodes/expression.py	96.78% <0.00%> (+0.03%)	⬆️
libcst/_nodes/op.py	95.73% <0.00%> (+0.07%)	⬆️
libcst/_nodes/base.py	95.48% <0.00%> (+0.07%)	⬆️
... and 2 more

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[zsol]

looks good, thanks for the contribution