Revert "drop py37 support"

[fselmo]

This reverts commit 82425694

What was wrong?

• There are breaking changes along with changes needed to keep in the next v2 minor release
• I needed to revert the breaking changes (drop py37 support) and keep the upper pin on hexbytes
• Bring back more breaking changes for python 3.7 like using importlib.metadata for version, pkg_resources...
• We will then need to add a lower pin to the master branch and release a new major version with this lower pin and without the upper pin present in this v2 branch

Todo:

• [x] Clean up commit history
• [x] Remove #144 entry from the release notes

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