Extend plugin version ranges down for Python 3.7

This uses narrow ranges, rather than pinning single versions, for
two of the Sphinx plugins whose versions are recently pinned.

This is so that plugin versions compatible with Python 3.7 (which
GitPython still supports) can be selected.

doc/requirements.txt

@@ -1,8 +1,8 @@
-sphinx==4.3.0
+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
+sphinxcontrib-applehelp >= 1.0.2, <= 1.0.4
+sphinxcontrib-devhelp == 1.0.2
+sphinxcontrib-htmlhelp >= 2.0.0, <= 2.0.1
+sphinxcontrib-qthelp == 1.0.3
+sphinxcontrib-serializinghtml == 1.1.5
 sphinx-autodoc-typehints