🔨 Update to use `build` for package building

[noahp]

Still using setuptools as the build backend, but switch to https://github.com/pypa/build for the frontend (it builds both sdist and wheel in the same command, and is PEP 517).

Several related updates:

• the scripts/dtrx file is moved to package layout (dtrx/dtrx.py), but is still runnable standalone
• reworking the tree to a python package style enables simpler and more typical package setup
• switch to setup.cfg, reduces the amount of messiness in the build configuration
• move the original documentation to archived/ to reduce confusion
• add some information about how the package is built + released to README.md
• add a Build step to the github actions

The setup.cfg file was formatted with the quite nice setup-cfg-fmt package (https://github.com/asottile/setup-cfg-fmt).

This also removes twine from the setup-requires dependencies. Fixes #19.

And moving the package configuration to setup.cfg, I noticed the subprocess32 backport dependency was not specified correctly when building the wheel (it was set based on the build environment, not the install environment), fixed now.

[noahp]

@ChrisJefferson Let me know what you think- this is a somewhat disruptive rework of the packaging scheme (still using setuptools though).

[ChrisJefferson]

Thanks, this looks really good.