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/inspect.py

@@ -45,6 +45,7 @@ import sys
 import tokenize
 import token
 import types
+import typing
 import warnings
 import functools
 import builtins
@@ -1877,7 +1878,10 @@ def _signature_is_functionlike(obj):
     code = getattr(obj, '__code__', None)
     defaults = getattr(obj, '__defaults__', _void) # Important to use _void ...
     kwdefaults = getattr(obj, '__kwdefaults__', _void) # ... and not None here
-    annotations = getattr(obj, '__annotations__', None)
+    try:
+        annotations = _get_type_hints(obj)
+    except AttributeError:
+        annotations = None
 
     return (isinstance(code, types.CodeType) and
             isinstance(name, str) and
@@ -2118,6 +2122,16 @@ def _signature_fromstr(cls, obj, s, skip_bound_arg=True):
 
     return cls(parameters, return_annotation=cls.empty)
 
+def _get_type_hints(func):
+    try:
+        return typing.get_type_hints(func)
+    except Exception:
+        # First, try to use the get_type_hints to resolve
+        # annotations. But for keeping the behavior intact
+        # if there was a problem with that (like the namespace
+        # can't resolve some annotation) continue to use
+        # string annotations
+        return func.__annotations__
 
 def _signature_from_builtin(cls, func, skip_bound_arg=True):
     """Private helper function to get signature for
@@ -2161,7 +2175,8 @@ def _signature_from_function(cls, func, skip_bound_arg=True):
     positional = arg_names[:pos_count]
     keyword_only_count = func_code.co_kwonlyargcount
     keyword_only = arg_names[pos_count:pos_count + keyword_only_count]
-    annotations = func.__annotations__
+    annotations = _get_type_hints(func)
+
     defaults = func.__defaults__
     kwdefaults = func.__kwdefaults__