MAINT: Dont import for version

[larsoner]

After #381 we starting hitting errors in CIs in MNE-Python when doing pip install -r requirements_doc.txt:

Collecting https://github.com/numpy/numpydoc/archive/main.zip (from -r requirements_doc.txt (line 3))
  Downloading https://github.com/numpy/numpydoc/archive/main.zip
  Preparing metadata (setup.py): started
  Preparing metadata (setup.py): finished with status 'error'
  error: subprocess-exited-with-error
  
  python setup.py egg_info did not run successfully.
  exit code: 1
  
  [10 lines of output]
  Traceback (most recent call last):
    File "<string>", line 2, in <module>
    File "<pip-setuptools-caller>", line 34, in <module>
    File "C:\Users\VssAdministrator\AppData\Local\Temp\pip-req-build-ww_dbswd\setup.py", line 6, in <module>
      from numpydoc import __version__ as version
    File "C:\Users\VssAdministrator\AppData\Local\Temp\pip-req-build-ww_dbswd\numpydoc\__init__.py", line 30, in <module>
      _verify_sphinx_jinja()
    File "C:\Users\VssAdministrator\AppData\Local\Temp\pip-req-build-ww_dbswd\numpydoc\__init__.py", line 17, in _verify_sphinx_jinja
      import sphinx, jinja2
  ModuleNotFoundError: No module named 'sphinx'

In other words, now import numpydoc does a check having to do with Sphinx, but this is a problem because setup.py imports numpydoc, so setup.py can't be run without Sphinx present.

This works around the issue by using a _version.py, that way numpydoc doesn't have to be imported during setup.py.

[larsoner]

This is a quick hotfix for a new bug appearing in the wild and it's green so I'll go ahead and merge. But anyone feel free to review and I'm happy to revisit / revise in another PR!