Make it build on pip 24.x

[mkmik]

Under recent versions of pip, the pygments-shader package can't be installed because the setup.py loads a dependency before setup() has executed:

$ pip install -e .
....
        File "<string>", line 3, in <module>
        File "/private/tmp/pygments-shader/pygments_shader/__init__.py", line 3, in <module>
          from .lexer import *
        File "/private/tmp/pygments-shader/pygments_shader/lexer.py", line 1, in <module>
          from pygments.lexer import RegexLexer, bygroups, using, this, words
      ModuleNotFoundError: No module named 'pygments'
      [end of output]

The simple fix would be to just simplify the code and just use version='1.0.1' in the setup.py file.

However, that could potentially break users of the pygments-shader package that due to Hyrum's law could depend on the ability to refer to the pygments_shader.__version__ variable.

So instead, this PR uses SourceFileLoader to load version.py from setup.py without also importing other stuff that is not yet ready. (thanks to @Guillemdb for teaching me that trick)