Prevent Sphinx warning about language = None
Since version 5.x Sphinx triggers a warning when `language = None`, this
caused `tox -e docs` to fail.
Set it to "en".
See
@@ -94,7 +94,7 @@ version = release = find_version("..", "vcr", "__init__.py")
#
# This is also used if you do content translation via gettext catalogs.
# Usually you set "language" from the command line for these cases.
-language = None
+language = "en"
# There are two options for replacing |today|: either, you set today to some
# non-false value, then it is used:
