Pin Sphinx plugins to compatible versions

This fixes version incompatibilities with major version 4 of
Sphinx, which GitPython is still using for the time being. Some
plugins that previously had depended back on Sphinx have had those
dependencies removed to avoid dependency cycles, but the effect is
that `pip` no longer is aware of or able to enforce version
compatibility, and newer plugin versions than can actually run with
Sphinx 4 are installed instead of older, usable versions.

This fixes the problem by pinning each of the affected Sphinx
plugins' latest actually compatible versions, i.e., latest versions
that do not need Sphinx 5 or higher.

The alternative of moving to Sphinx 5 or higher should probably be
done eventually, but will require addressing a cross-reference
ambiguity in type annotations for the `Actor` class.

For details on the bug, see:

- #1799 (comment)
- #1802

doc/requirements.txt

@@ -1,3 +1,8 @@
 sphinx==4.3.0
 sphinx_rtd_theme
+sphinxcontrib-applehelp==1.0.4
+sphinxcontrib-devhelp==1.0.2
+sphinxcontrib-htmlhelp==2.0.1
+sphinxcontrib-qthelp==1.0.3
+sphinxcontrib-serializinghtml==1.1.5
 sphinx-autodoc-typehints