Replace broken imports from version file in setup script

[mdickinson]

Closes #178

To verify the fix (on a Unix-a-like platform), execute the following instructions from the repository root, replacing the build number in the sdist as appropriate:

python -m venv --clear scimath_env
source scimath_env/bin/activate
pip install build
python -m build --sdist
cd dist
pip install scimath-5.1.0.dev423.tar.gz

The above works for me on this branch, but on main the final pip install step fails with an error resembling that reported in #178:

(scimath_env) mdickinson@mirzakhani dist % pip install scimath-5.1.0.dev423.tar.gz
WARNING: Requirement 'scimath-5.1.0.dev423.tar.gz' looks like a filename, but the file does not exist
Processing ./scimath-5.1.0.dev423.tar.gz
ERROR: Could not install packages due to an OSError: [Errno 2] No such file or directory: '/Users/mdickinson/Enthought/ETS/scimath/dist/scimath-5.1.0.dev423.tar.gz'

(scimath_env) mdickinson@mirzakhani dist % ls
scimath-5.1.0.dev422.tar.gz
(scimath_env) mdickinson@mirzakhani dist % pip install scimath-5.1.0.dev422.tar.gz 
Processing ./scimath-5.1.0.dev422.tar.gz
  Installing build dependencies ... done
  Getting requirements to build wheel ... error
  error: subprocess-exited-with-error
  
  × Getting requirements to build wheel did not run successfully.
  │ exit code: 1
  ╰─> [25 lines of output]
      Traceback (most recent call last):
        File "<string>", line 86, in write_version_py
      ModuleNotFoundError: No module named 'scimath'
      
      During handling of the above exception, another exception occurred:
      
      Traceback (most recent call last):
        File "/Users/mdickinson/Enthought/ETS/scimath/scimath_env/lib/python3.12/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 353, in <module>
          main()
        File "/Users/mdickinson/Enthought/ETS/scimath/scimath_env/lib/python3.12/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 335, in main
          json_out['return_val'] = hook(**hook_input['kwargs'])
                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        File "/Users/mdickinson/Enthought/ETS/scimath/scimath_env/lib/python3.12/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 118, in get_requires_for_build_wheel
          return hook(config_settings)
                 ^^^^^^^^^^^^^^^^^^^^^
        File "/private/var/folders/07/jbbjv8b53bs5y9xyjdyn1zgw0000gn/T/pip-build-env-sx5r8mb8/overlay/lib/python3.12/site-packages/setuptools/build_meta.py", line 325, in get_requires_for_build_wheel
          return self._get_build_requires(config_settings, requirements=['wheel'])
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        File "/private/var/folders/07/jbbjv8b53bs5y9xyjdyn1zgw0000gn/T/pip-build-env-sx5r8mb8/overlay/lib/python3.12/site-packages/setuptools/build_meta.py", line 295, in _get_build_requires
          self.run_setup()
        File "/private/var/folders/07/jbbjv8b53bs5y9xyjdyn1zgw0000gn/T/pip-build-env-sx5r8mb8/overlay/lib/python3.12/site-packages/setuptools/build_meta.py", line 311, in run_setup
          exec(code, locals())
        File "<string>", line 122, in <module>
        File "<string>", line 89, in write_version_py
      ImportError: Unable to import git_revision. Try removing scimath/_version.py and the build directory before building.
      [end of output]
  
  note: This error originates from a subprocess, and is likely not a problem with pip.
error: subprocess-exited-with-error

× Getting requirements to build wheel did not run successfully.
│ exit code: 1
╰─> See above for output.

note: This error originates from a subprocess, and is likely not a problem with pip.

[mdickinson]

I don't think we need this any more: it may have been relevant when there were version files around that didn't include the git revision, but at this point it's safe to assume that if the version file exists in the expected location, then it was generated by this setup script and does include the git revision.

[mdickinson]

Closing and re-opening to re-trigger the EDM runs.