Fix SEGFAULT from calling FT_New_Face/FT_Done_Face in multiple threads

[lysnikolaou]

Fixes #8454.

Changes proposed in this pull request:

• Introduce a mutex that needs to be held every time FT_New_Face or FT_Done_Face are called.
• Upgrade pythoncapi_compat.h to the latest version.

[hugovk]

Thanks!

[nulano]

Is anything here actually using the new changes in pythoncapi_compat.h?

There's no reason to stick with an old version, but I'm curious if there is a reason for updating it in what looks like an unrelated PR?

[lysnikolaou]

Is anything here actually using the new changes in pythoncapi_compat.h?

There's no reason to stick with an old version, but I'm curious if there is a reason for updating it in what looks like an unrelated PR?

No, upgrading pythoncapi_compat was actually needed in an earlier version of my patch, but I actually ended up not needing the new stuff.

However, I thought that it might be okay for the header to be upgraded anyway, that's why I left it in.