Build updates and modernization

[mdickinson]

This PR updates the build machinery to use pyproject.toml, and drops Python 3.6 support.

Detailed changes

• Drop Python 3.6 support; we now require Python >= 3.7.
• Drop Python 3.6 from test workflows.
• Only use PySide6 for testing. (The main package still permits PySide2 and PyQt5 for Python versions for which they're available; we just don't test on those.)
• Rename setup.cfg (which only contained flake8 configuration settings) to .flake8
• Replace setup.py with pyproject.toml
• Remove various outdated references to setup.py
• Remove envisage.version and envisage.__version__ (and tests)
• Optional dependencies (a.k.a. extras_require) have been removed
• We no longer rely on ETS_TOOLKIT (or QT_API) being set in order to run tests that require a GUI; we just rely on Pyface's default mechanisms for toolkit detection. (As a consequence, we retain the option to use ETS_TOOLKIT if it's helpful.)
• etstool.py support for backends other than those available through EDM has been removed. Meanwhile, support for pyqt6 has been added. (Tests currently fail with PyQt6, due to a known issue in TraitsUI: https://github.com/enthought/traitsui/issues/1926)

Closes #370 Closes #511 Closes #512 Closes #519 Closes #525

[mdickinson]

@corranwebster This is ready for review, if you have the bandwidth available.

[mdickinson]

This is ready for review, if you have the bandwidth available.

Sorry, I take that back. The juggling with the changes in #526 is getting annoying; I think it makes most sense to merge that PR into this one. I'll change back to draft while that's in progress.