Update repository for newer Pythons, packaging standards

[mcflugen]

@awickert This pull request updates the gflex repository for Python 3.10+ and also updates the project to follow standard packaging conventions.

Highlights:

• Updated for Python 3.12 (dropped support for Python 2)
• Moved static project metadata from seutp.py into pyproject.toml
• Set up CI to run on GitHub actions (remove travis)
• Refactored the gflex cli to use argparse
• Added __main__.py to run gflex as a module
• Added a changelog
• Added a nox file to automate routine maintenance tasks (e.g. running the tests, running the linters, uploading to PyPI, etc.)
• Added a pre-commit config file so that the linters can be run using pre-commit
• Reformatted code to follow a consistent style
• Removed lint

Because I've reformatted the code to use a consistent style, just looking at the difference between this branch and master will likely show too many changes to be useful. If you want to have a closer look, it might be easier to look at individual commits.

I haven't made any meaningful changes within the actual gflex code, so things should work exactly as before.

[mcflugen]

@awickert One thing I could use some help with are some remaining bare try/except blocks. I fixed ones that looked obvious to me, for example things like,

try:
    self.x
except:
    self.x = 0.0

to

try:
    self.x
except AttributeError:
    self.x = 0.0

There are some other blocks, though, that I wasn't totally sure what exception you were expecting they might raise. Because a bare except block will catch all errors—including ones you don't intend—it's a good idea to explicitly state what exception you are trying to catch and to include the absolute minimum amount of code in the try block.

[awickert]

Thank you, Eric. I just merged everything and will run some basic tests.

This was a LARGE amount of work on your end, and I appreciate it!

If you remember, could you create an "Issue" with your follow-up comment here? That will make sure that it jogs my memory, and it would be helpful to me if it's attributed to you (otherwise I would just add it myself).

Thanks!