Intern strings for performance

[qbx2]

Hello. pyo3-asyncio should apply pyo3::intern! to all constant strings to avoid duplicated string copy and unicode validation.

Example usages:

• https://github.com/awestlake87/pyo3-asyncio/blob/0ce6d6dbad3668f98553f0d3b9c350e5f42a1996/src/lib.rs#L415
• https://github.com/awestlake87/pyo3-asyncio/blob/0ce6d6dbad3668f98553f0d3b9c350e5f42a1996/src/generic.rs#L566

[awestlake87]

Looks like PyO3 v0.20 just came out a couple hours ago, so this next release might be a good opportunity to try it out. Thanks for the suggestion!