Bump and migrate pyo3 to 0.21

[jopemachine]

Thank you for contributing to pyo3-asyncio!

Please consider adding the following to your pull request:

• an entry in CHANGELOG.md
• docs to all new functions and / or detail in the guide
• tests for all new or changed functions

Be aware the CI pipeline will check your pull request for the following:

• Rust tests (Just cargo test)
• Rust lints (make clippy)
• Rust formatting (cargo fmt)
• Python formatting (black . --check. You can install black with pip install black)

Fixes #119.

Ref: https://pyo3.rs/v0.21.0/migration.html

I've updated pyo3-asyncio code referring to the migration guide.
I'd appreciate it if you could let me know if there's anything I've missed.

[MeepoWin]

Any progress of this? It sort of block our migration.

[cjdsellers]

@awestlake87 Are there any subtasks here you need a hand with?

[cjdsellers]

@jopemachine I think the changes for behavior (American English) -> behaviour (British English) is another case of words which aren't actually typos, we might want to revert those too to make the PR easier to review?

[jopemachine]

@jopemachine I think the changes for behavior (American English) -> behaviour (British English) is another case of words which aren't actually typos, we might want to revert those too to make the PR easier to review?

It is already reverted in df5f892 (Reverted words misunderstood as typo)

[cjdsellers]

I'm still seeing changes at the head of this branch for Code-of-Conduct.md though?

I must be misunderstanding - this PR is also reverting previous changes which were considered typos I'm assuming.

[kdmkone]

Hi! Is there anything we can do to help port this over? We're currently blocked in migrating to a newer PyO3 version for this.

[davidhewitt]

As per #119 (comment)

While waiting for a permanent decision on what to do, I've published https://crates.io/crates/pyo3-asyncio-0-21 from a fork at https://github.com/davidhewitt/pyo3-asyncio