Replace use of pkg_resources with importlib_metadata

[corranwebster]

Python 3.8 introduces standard library code for entrypoint handling via the importlib.metadata module. This is back-ported to earlier versions of Python via importlib_metadata.

This should give better forward compatibility, and may help people who want to freeze Pyface/TraitsUI apps.

[rahulporuri]

Using importlib.metadata instead of pkg_resources also improves startup time.

❯ python -Ximporttime -c "import importlib.metadata"
import time: self [us] | cumulative | imported package
...
import time:      6528 |     141065 | importlib.metadata

❯ python -Ximporttime -c "import pkg_resources"
import time: self [us] | cumulative | imported package
...
import time:     28439 |     441931 | pkg_resources

I consistently see 400-450 ms for pkg_resources vs 100-150 ms for importlib.metadata. Note that this is with python 3.8.6. I mostly see the same numbers on python 3.9.

[rahulporuri]

@corranwebster i've added this to the Q4 2020 sprint backlog, in hopes that we can find time to address this before the end of the quarter.

[mdickinson]

Current uses of pkg_resources fall into two different categories: resource location and entry-point enumeration - we'll need importlib.resources rather than importlib.metadata for the first category.

Last time we looked at importlib.resources, it had issues (namely that data directories had to be full-blown Python packages for things to work). That's changed since Python 3.9, and in the importlib_resources backport, so it may be worth another look.

Related issue: https://gitlab.com/python-devs/importlib_resources/-/issues/58

[kitchoi]

#790 only resolves half of the usage of pkg_resources, we still have #791 pending.

[rahulporuri]

closing as #791 is also merged.