Add python source to `Cargo.toml`

[Quexington]

I made this because it allows me to specify a specific commit to install chia_rs from with pip. Without this entry, the stub file doesn't come along and mypy doesn't function properly. The chia-blockchain CI has some difficulties with this but I think that's because it is manually building it when installing from the commit rather than installing from a published version. I'm not entirely sure how to check that though so some input on how reasonable this seems would be appreciated.

[Rigidity]

Source installations via pip working is blocked on this issue: https://github.com/Chia-Network/chia_rs/issues/349

[Quexington]

Source installations via pip working is blocked on this issue: #349

I don't follow this thread so well this is not my domain of expertise but I don't think this requires anything new to happen during your run of the mill build process, it just enables pip installing from source to work (on at least some platforms). I don't know though.

[coveralls-official[bot]]

Pull Request Test Coverage Report for Build 9669819579

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 287 unchanged lines in 10 files lost coverage.
• Overall coverage increased (+2.0%) to 80.756%

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Files with Coverage Reduction	New Missed Lines	%
crates/chia-protocol/src/classgroup.rs	1	0.0%
crates/chia-bls/src/error.rs	4	0.0%
crates/chia-traits/src/streamable.rs	9	92.54%
crates/chia-bls/src/secret_key.rs	11	89.95%
crates/chia-bls/src/signature.rs	19	93.94%
crates/chia-bls/src/public_key.rs	19	90.45%
wheel/src/api.rs	43	66.67%
crates/chia-bls/src/gtelement.rs	47	9.47%
crates/chia-bls/src/bls_cache.rs	51	73.16%
crates/chia-protocol/src/block_record.rs	83	0.0%
<!--	Total:	287		-->

Totals
Change from base Build 9645397669:	2.0%
Covered Lines:	11393
Relevant Lines:	14108

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💛 - Coveralls

[Quexington]

As long as no one sees any unexpected side effects of this then go ahead and merge it at your convenience