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:

• pyo3-asyncio/src/lib.rs

  Line 415 in 0ce6d6d

  event_loop.call_method0("create_future")

• pyo3-asyncio/src/generic.rs

  Line 566 in 0ce6d6d

  "add_done_callback",

[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!