Let dependencies in the pyproject.toml float upwards to reduce depencency version conflicts

[uckelman-sf]

Let dependencies in the pyproject.toml float upwards to reduce conflicts when using capa as a library. Added requirements.txt to continue pinning dependency versions for release builds.

Resolves #2053.

Checklist

• [ ] No CHANGELOG update needed

• [X] No new tests needed

• [ ] No documentation update needed

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[sf-jonstewart]

Howdy! I... reaaaaaaaaaaalllllllllllllly... don't want to take three months of conference calls to sign a stupid CLA for some greater-than signs. Please consider this a well-specified bug report. :-)