imp module is removed in python3.12

[drew-parsons]

Describe the bug
The sasview doc build uses the imp module to import sources,

sasview/docs/sphinx-docs/build_sphinx.py

Line 15 in fe8ec80

import imp

used at

sasview/docs/sphinx-docs/build_sphinx.py

Line 66 in fe8ec80

run = imp.load_source('run', joinpath(SASVIEW_ROOT, 'run.py'))

The imp module is deprecated and removed from python3.12, so sasview fails to build on python 3.12 (or at least docs fail to build)

build_sphinx.py needs to be updated to use importlib instead.
It probably needs little more than finding the replacement for imp.load_source

To Reproduce
Steps to reproduce the behavior:

1. cd docs/sphinx-docs
2. make -Cdocs/sphinx-docs html

Expected behavior
Doc (and sasview itself) should build on python 3.12.

Screenshots
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1061760

 python3 setup.py docs update
running docs
========= check for sasmodels at /<<BUILDDIR>>/sasmodels/doc ============
== !!WARNING!! sasmodels directory not found. Cannot build model docs. ==
Traceback (most recent call last):
  File "/<<PKGBUILDDIR>>/setup.py", line 292, in <module>
    setup(
  File "/usr/lib/python3/dist-packages/setuptools/__init__.py", line 
107, in setup
    return distutils.core.setup(**attrs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/setuptools/_distutils/core.py", 
line 185, in setup
    return run_commands(dist)
           ^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/setuptools/_distutils/core.py", 
line 201, in run_commands
    dist.run_commands()
  File "/usr/lib/python3/dist-packages/setuptools/_distutils/dist.py", 
line 969, in run_commands
    self.run_command(cmd)
  File "/usr/lib/python3/dist-packages/setuptools/dist.py", line 1233, 
in run_command
    super().run_command(command)
  File "/usr/lib/python3/dist-packages/setuptools/_distutils/dist.py", 
line 988, in run_command
    cmd_obj.run()
  File "/<<PKGBUILDDIR>>/setup.py", line 106, in run
    import build_sphinx
  File "/<<PKGBUILDDIR>>/docs/sphinx-docs/build_sphinx.py", line 16, in 
<module>
    import imp
ModuleNotFoundError: No module named 'imp'
make[1]: *** [debian/rules:30: execute_after_dh_auto_build] Error 1
make[1]: Leaving directory '/<<PKGBUILDDIR>>'
make: *** [debian/rules:20: binary] Error 2

SasView version:

• Version:5.0.6, HEAD

Operating system (please complete the following information):

• OS: Linux (debian unstable)
• Python 3.12 (any OS)

Additional context
Affects update plans in #2546