Add a version menu to the Sphinx build (#394)

* Fix warnings in the Sphinx build

* Include the version number in the header

* Enable the version dropdown menu

To make this work, a versions.json file will need to be added to the root of
the gh-pages branch, which is a dict mapping "label": "path" for each version.
This should be updated whenever a new version is tagged.

spec/conf.py

@@ -62,6 +62,8 @@
     ('py:class', 'collections.abc.Sequence'),
     ('py:class', "Optional[Union[int, float, Literal[inf, - inf, 'fro', 'nuc']]]"),
     ('py:class', "Union[int, float, Literal[inf, - inf]]"),
+    ('py:obj', "typing.Optional[typing.Union[int, float, typing.Literal[inf, - inf, 'fro', 'nuc']]]"),
+    ('py:obj', "typing.Union[int, float, typing.Literal[inf, - inf]]"),
     ('py:class', 'PyCapsule'),
     ('py:class', 'enum.Enum'),
     ('py:class', 'ellipsis'),
@@ -123,7 +125,7 @@
 html_theme_options = {
 
     # Set the name of the project to appear in the navigation.
-    'nav_title': 'Python array API standard',
+    'nav_title': f'Python array API standard {release}',
 
     # Set you GA account ID to enable tracking
     #'google_analytics_account': 'UA-XXXXX',
@@ -170,8 +172,8 @@
         #"customization": "Configuration options to personalize your site.",
     },
 
-    #"version_dropdown": True,
-    #"version_json": "_static/versions.json",
+    "version_dropdown": True,
+    "version_json": "../versions.json",
     "table_classes": ["plain"],
 }