bpo-38605: Make 'from __future__ import annotations' the default (GH-20434)

The hard part was making all the tests pass; there are some subtle issues here, because apparently the future import wasn't tested very thoroughly in previous Python versions.

For example, `inspect.signature()` returned type objects normally (except for forward references), but strings with the future import. We changed it to try and return type objects by calling `typing.get_type_hints()`, but fall back on returning strings if that function fails (which it may do if there are future references in the annotations that require passing in a specific namespace to resolve).

Lib/dataclasses.py

@@ -399,8 +399,10 @@ def _create_fn(name, args, body, *, globals=None, locals=None,
 
     ns = {}
     exec(txt, globals, ns)
-    return ns['__create_fn__'](**locals)
-
+    func = ns['__create_fn__'](**locals)
+    for arg, annotation in func.__annotations__.copy().items():
+        func.__annotations__[arg] = locals[annotation]
+    return func
 
 def _field_assign(frozen, name, value, self_name):
     # If we're a frozen class, then assign to our fields in __init__
@@ -651,6 +653,11 @@ def _is_type(annotation, cls, a_module, a_type, is_type_predicate):
     # a eval() penalty for every single field of every dataclass
     # that's defined.  It was judged not worth it.
 
+    # Strip away the extra quotes as a result of double-stringifying when the
+    # 'annotations' feature became default.
+    if annotation.startswith(("'", '"')) and annotation.endswith(("'", '"')):
+        annotation = annotation[1:-1]
+
     match = _MODULE_IDENTIFIER_RE.match(annotation)
     if match:
         ns = None
@@ -991,7 +998,7 @@ def _process_class(cls, init, repr, eq, order, unsafe_hash, frozen):
     if not getattr(cls, '__doc__'):
         # Create a class doc-string.
         cls.__doc__ = (cls.__name__ +
-                       str(inspect.signature(cls)).replace(' -> None', ''))
+                       str(inspect.signature(cls)).replace(' -> NoneType', ''))
 
     abc.update_abstractmethods(cls)