Add __version__

[ekiefl]

I just updated my arcadia-pycolor version but besides checking my lock file I had no good way to confirm I was importing the correct version.

It is standard to put a package's version in the top-level __init__.py under the variable __version__:

>>> import importlib
>>> importlib.import_module("numpy").__version__
'1.26.4'
>>> importlib.import_module("pandas").__version__
'2.2.2'
>>> importlib.import_module("biotite").__version__
'0.40.0'
>>> importlib.import_module("dask").__version__
'2024.6.2'
>>> importlib.import_module("arcadia_pycolor").__version__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'arcadia_pycolor' has no attribute '__version__'

This PR adds __version__. Since we are now between 0.5.1 and 0.5.2, I've appended a pre-release identifier -dev. We could start doing -dev.X, -rc.X, -alpha.X, -beta.X, etc, but IMO this package doesn't require anything more complex than -dev.

[keithchev]

Oops, yes, totally. This is my bad. I think the best way to do this is probably to use dynamic versioning, so that we can create new a release simply by creating a new tag on Github. This will avoids the need to hard-code the version anywhere.

[keithchev]

Update: I've added dynamic versioning in #48.

[ekiefl]

Superseded by #48