inspect.get_annotations(o) safely retrieves and return
 the annotations from o.  It handles un-stringizing
stringized annotations, it works around the quirks of
annotations on various objects, and it is super-safe.

inspect.signature() now calls inspect.get_annotations()
to get the annotations for an object.

Thank goodness!

Thanks, Fidget_Spinner!

If an explicit locals dict is not passed in, get_annotations(cls)
will now use the cls namespace as locals by default.

inspect.get_annotations() and inspect.signature() will now
only call eval() on string values by explicit user request.
Calling eval() by default is too opinionated for the standard
library.

Neither function catches exceptions, so if the eval() raises
an exception, it'll unwind up to the caller.

Also, removed the special eval_str=ONLY_IF_STRINGIZED logic
entirely.  The user now tells us explicitly whether or not
to call eval() on strings--so we don't need the heuristic
for anything.

inspect.signature() and inspect.Signature_from_callable()
both call inspect.get_annotations() to compute the annotations.
Both functions also pass through their globals, locals, and
eval_str arguments.  This commit adds some tests just to
ensure that the values are passed through correctly and everything
is working.  And who doesn't feel more secure with some extra tests!

__annotations__ is allowed to be None in some circumstances,
and there is explicit code to handle it in the inspect module.
But I introduced a bug ("isinstance(o, None)"), and this showed
the code path wasn't being executed anyway, so now there's a
bugfix and a test too.