feat: add PEP 621 support (flit and trampolim)

[henryiii]

Added PEP 621 support for Flit and added trampolim.

• Flit SDists have bad dist-info files that produces twine warnings, but that's not new.
• Trampolim products don't pass twine due to twine not supporting metadata 2.2 - skipping check
  • Needed a duplicate CI file due to upload action checking by default - also added "pre"-check for non-release runs.
• Trampolim doesn't support /src structure (or editable installs) yet (at least src should come soon, thanks to @FFY00), so that one does not make /src structure for now. Support for src structure FFY00/trampolim#3
• Trampolin is Python 3.7+, but wheels should support 3.6+, I'd think.

[henryiii]

Details:

• Trampolim products don't pass twine due to twine not supporting metadata 2.2 - skipping check
  • Needed a duplicate CI file due to upload action checking by default - also added "pre"-check for non-release runs.
• Flit SDists have bad dist-info files that produces twine warnings, but that's not new.
• Trampolim doesn't support /src structure (or editable installs) yet (at least src should come soon, thanks to @FFY00), so that one does not make /src structure for now. Support for src structure FFY00/trampolim#3