Remove usage of pkg_resources

[felixonmars]

pkg_resources is expensive to load, which takes more than 400ms here.

Let's use importlib.metadata from stdlib for Python 3.8+, and fallback to importlib_metadata for Python 3.7.

Note that setuptools was a missing runtime dependency before this change.

This accelerates startup time significantly:

Before:

$ time latex2mathml -V
latex2mathml 3.73.1

real    0m0.728s
user    0m0.607s
sys     0m0.113s

After:

$ time latex2mathml -V
latex2mathml 3.73.1

real    0m0.156s
user    0m0.121s
sys     0m0.037s

[codecov[bot]]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (fc18bdb) compared to base (60b9563). Patch has no changes to coverable lines.

Additional details and impacted files

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