Bump importlib metadata dependency

[jaraco]

• Consolidate importlib metadata import
• Rely on importlib_metadata for Python 3.9 and earlier. Fixes InvalidRequirement on Python 3.8 #630.

[layday]

You'll also need to update the min version in https://github.com/pypa/build/blob/main/tests/constraints.txt. Technically, the PR looks solid - we should've consolidated the metadata import in a separate module long ago :) I don't know if we wanna add a fallback for packagers however.

[henryiii]

FYI, personally, I have started putting all backports into _compat, with the same folder structure as the stdlib. So this would be _compat/importlib.py or _compat/importlib/metadata.py. Don't have to follow that here, I just find it works well, scales to multiple backports, and is easy to remove when the backport period is over.

I don't know if we wanna add a fallback for packagers however.

Guessing you mean to make bootstrapping, etc easier? So we'd have a try/except around the <3.10 branch?

[jaraco]

FYI, personally, I have started putting all backports into _compat, with the same folder structure as the stdlib. So this would be _compat/importlib.py or _compat/importlib/metadata.py. Don't have to follow that here, I just find it works well, scales to multiple backports, and is easy to remove when the backport period is over.

I often do something similar, and I go a step further and group the compat modules by Python version, so py38compat supplies compatibility interfaces until Python 3.8 support is dropped (example). That makes it really easy to locate and remove the relevant pieces when they're no longer needed.

[jaraco]

You'll also need to update the min version in https://github.com/pypa/build/blob/main/tests/constraints.txt.

Done in 72537a7.