py domain: Suppress the leading "typing." module from typehints

To support creating hyperlinks to container types naturally, py domain
should take fully-qualified typehints for them.  But nobody wants to
show "typing." module name on the signature. So this suppresses them
automatically.

sphinx/domains/python.py

@@ -105,6 +105,8 @@ def type_to_xref(target: str, env: BuildEnvironment = None, suppress_prefix: boo
         text = target.split('.')[-1]
     elif suppress_prefix:
         text = target.split('.')[-1]
+    elif target.startswith('typing.'):
+        text = target[7:]
     else:
         text = target
 
@@ -1488,7 +1490,7 @@ def istyping(s: str) -> bool:
         return None
     elif node.get('reftype') in ('class', 'obj') and node.get('reftarget') == 'None':
         return contnode
-    elif node.get('reftype') in ('class', 'exc'):
+    elif node.get('reftype') in ('class', 'obj', 'exc'):
         reftarget = node.get('reftarget')
         if inspect.isclass(getattr(builtins, reftarget, None)):
             # built-in class

tests/test_domain_py.py

@@ -350,7 +350,7 @@ def test_parse_annotation(app):
 
     # Literal type makes an object-reference (not a class reference)
     doctree = _parse_annotation("typing.Literal['a', 'b']", app.env)
-    assert_node(doctree, ([pending_xref, "typing.Literal"],
+    assert_node(doctree, ([pending_xref, "Literal"],
                           [desc_sig_punctuation, "["],
                           [desc_sig_literal_string, "'a'"],
                           [desc_sig_punctuation, ","],
@@ -384,7 +384,7 @@ def test_parse_annotation_Literal(app):
                           [desc_sig_punctuation, "]"]))
 
     doctree = _parse_annotation("typing.Literal[0, 1, 'abc']", app.env)
-    assert_node(doctree, ([pending_xref, "typing.Literal"],
+    assert_node(doctree, ([pending_xref, "Literal"],
                           [desc_sig_punctuation, "["],
                           [desc_sig_literal_number, "0"],
                           [desc_sig_punctuation, ","],