Don't support building documentation on Python 3.7

This removes the specially cased alternative lower versions of
`sphinx` and its dependencies that, since gitpython-developers#1954, were only for
Python 3.7. As discussed in comments there, this simplifies the
documentation dependencies and avoids a situation where the version
of Python used to build the documentation has a noticeable effect
on the generated result.

This also conditions running the "Documentation" step in the main
CI test workflow (`pythonpackage.yml`) on the Python version not
being 3.7 (otherwise the job would always fail).

The only change this makes to the support status of GitPython on
Python 3.7 is to no longer support building documentation on 3.7.
GitPython can still be installed and used on 3.7 (though usually
this would not be a good idea, outside of testing, since Python 3.7
itself has not been supported by the Python Software Foundation for
quite some time). In addition, the documentation, which can be
built on any version >= 3.8 (including 3.13 starting in gitpython-developers#1954) is
no less relevant to usage on Python 3.7 than it was before.

.github/workflows/pythonpackage.yml

@@ -99,6 +99,7 @@ jobs:
       continue-on-error: false
 
     - name: Documentation
+      if: matrix.python-version != '3.7'
       run: |
         pip install ".[doc]"
         make -C doc html

doc/requirements.txt

@@ -1,9 +1,3 @@
-sphinx >= 7.1.2, < 7.2 ; python_version >= "3.8"
-sphinx == 4.3.2 ; python_version < "3.8"
-sphinxcontrib-applehelp >= 1.0.2, <= 1.0.4 ; python_version < "3.8"
-sphinxcontrib-devhelp == 1.0.2 ; python_version < "3.8"
-sphinxcontrib-htmlhelp >= 2.0.0, <= 2.0.1 ; python_version < "3.8"
-sphinxcontrib-qthelp == 1.0.3 ; python_version < "3.8"
-sphinxcontrib-serializinghtml == 1.1.5 ; python_version < "3.8"
+sphinx >= 7.1.2, < 7.2
 sphinx_rtd_theme
 sphinx-autodoc-typehints