Don't support building documentation on Python 3.8

This discontinues supporting building documentation on Python 3.8.
It does not affect installing or running GitPython on Python 3.8
(except when the `doc` extra is used, but this is only used for
building documentation). The reason is that it is no longer
possible to use the same version of Sphinx on Python 3.8 as on the
most recent supported versions of Python, because Python 3.14 no
longer has `ast.Str` (using `str.Constant` for string literals
instead), which causes the oldest version of `sphinx` that runs on
Python 3.14 to be `sphinx` 7.2.0, while the newest version that is
installable on Python 3.8 is `sphinx` 7.1.2.

The immediately preceding commit changes the requirements for the
`doc` extra to specify a newer `sphinx` version for Python 3.9 and
later. This can't be done on Python 3.8. Because there can be
subtle differences in documentation generated with different
`sphinx` versions, and because Python 3.8 has been end-of-life for
some time, it is not really worth carrying conditional dependencies
for the `sphinx` version in `doc/requirements.txt`.

Note that, while it is probably not a very good idea to use
GitPython (or anything) on Python 3.8 since it is end-of-life, this
change does not stop supporting installing GitPython on that or any
other version it has been supporting. Installing and using
GitPython remains supported all the way back to Python 3.7 at this
time. This only affects the `doc` extra and its requirements.

This change is analogous to the change made in #1956, which
followed up on the change in #1964 in the same way this change
follows up on the change in the immediately preceding commit.

Author: EliahKagan

.github/workflows/pythonpackage.yml

@@ -30,6 +30,11 @@ jobs:
         - os-type: ubuntu
           python-version: "3.7"
           os-ver: "22.04"
+        - build-docs: true  # We ensure documentation builds, except on very old interpreters.
+        - python-version: "3.7"
+          build-docs: false
+        - python-version: "3.8"
+          build-docs: false
         - experimental: false
 
       fail-fast: false
@@ -110,7 +115,7 @@ jobs:
       continue-on-error: false
 
     - name: Documentation
-      if: matrix.python-version != '3.7'
+      if: matrix.build-docs
       run: |
         pip install '.[doc]'
         make -C doc html

doc/requirements.txt

@@ -1,4 +1,3 @@
-sphinx >= 7.4.7, < 8 ; python_version >= "3.9"
-sphinx >= 7.1.2, < 7.2 ; python_version < "3.9"
+sphinx >= 7.4.7, < 8
 sphinx_rtd_theme
 sphinx-autodoc-typehints