Release 0.13

[davidhewitt]

Closes #1306

Unless I'm given a reason to wait, I'll release this in two days' time.

I suggest the following release text:

---

The major feature for this PyO3 release is support for the CPython limited API. This is an opt-in feature when building extension modules which makes it possible for the built wheel to be installable on multiple Python versions.

As well as this, a number of other refinements, fixes, and performance improvements have been merged into this release. In particular, note that the internal "macros" crates have been renamed to pyo3-macros and pyo3-macros-backend for clarity and consistency (previously pyo3cls and pyo3-derive-backend).

For full details of all changes, see the CHANGELOG.

Thank you to everyone who contributed code, documentation fixes, design ideas, bug reports, and feedback. The following users' commits are included in this release:

@AleksaC
@alex
@Askaholic
@aviramha
@birkenfeld
@daggy1234
@davidhewitt
@dvermd
@kngwyu
@konstin
@Palladinium
@roblabla
@scalexm
@Stranger6667

[kngwyu]

Thanks!
You may want to note the renaming of proc-macro crates?

[davidhewitt]

👍 Edited the release text draft above to mention.

[kngwyu]

I also want to say thanks to everyone who contributed to this version. ❤️
Especially, thanks @alex for abi3 support and thanks @davidhewitt for your continuous efforts to keep this project solid!