Allow parsing source code directly

[bzoracler]

Resolves #21

Tests are part of python/mypy#21260

[ilevkivskyi]

Thanks for the PR! Me or Jukka will try to take a look next week. In the meantime, could you please resolve the merge conflict?

[bzoracler]

Implementation taken from https://pyo3.rs/main/doc/src/pyo3/exceptions.rs#802-811 (PR https://github.com/PyO3/pyo3/pull/5668/changes), because new_err_from_utf8 is only in pyo3 0.29 which is not released yet.

The test is here: python/mypy@ac275e4

[bzoracler]

I've hard-coded Python source type here, because mypy parsing functions never previously exposed an option to let the user treat source code directly as a .pyi stub source.

[bzoracler]

Hmm...there might be some unnecessary allocations that are preventable. I'm going to do some benchmarking with another implementation.

[bzoracler]

If we're willing to use &str in downstream functions instead of String (which IMO is fine), 30bd655 removes unnecessary allocations when working with Python builtins.bytes passed as source code. Microbenchmarks on a test machine (Intel Core i7-10750H CPU @ 2.60GHz, x86_64, 12 cores) show ~45% reduction in allocation during type conversion from PyBytes to &str (tested with 100 MB source code), and the type conversion itself runs about 6x faster (although execution speed is dominated by parsing, so this speedup doesn't matter as much).

Performance of builtins.str passed as source code does not seem to be improvable unless stable-abi is increased to ["pyo3/abi3-py310"], after which we can use this extraction function. I tried the same microbenchmarks after increasing the stable-abi, got similar allocation reduction and a huge speedup in type conversion (thousands of times faster, but again the execution time is dominated by parsing so doesn't really matter here).