Python typehints: support forward references

Typehints may be given as string literals: https://www.python.org/dev/peps/pep-0484/#forward-references
These are currently not evaluated and result in errors.
Example 1:
```

def test_typed_callable_string_hints(self):
def do_fn(element: 'int') -> 'typehints.List[str]':

return [[str(element)] * 2]

result = [1, 2] | beam.ParDo(do_fn)
self.assertEqual([['1',
'1'], ['2', '2']], sorted(result))

```

This results in:
```

> return issubclass(sub, base)
E TypeError: issubclass() arg 2 must be a class or tuple of
classes

typehints.py:1168: TypeError

```

Example 2:
```

def test_typed_dofn_string_hints(self):
class MyDoFn(beam.DoFn):
def process(self, element:
'int') -> 'typehints.List[str]':
return [[str(element)] * 2]

result = [1, 2] | beam.ParDo(MyDoFn())

self.assertEqual([['1', '1'], ['2', '2']], sorted(result))

```

This results in:
```

> raise ValueError('%s is not iterable' % type_hint)
E ValueError: typehints.List[str] is
not iterable

typehints.py:1194: ValueError

```

where the non-iterable entity the error refers to is a string literal ("typehints.List[str]").

Imported from Jira [BEAM-8487](https://issues.apache.org/jira/browse/BEAM-8487). Original Jira may contain additional context.
Reported by: udim.

Start in typehints.py around the failures at lines 1168 and 1194, using the two typed callable and DoFn examples as reproduction cases. Trace how string-literal annotations are handled, then confirm both examples work without the shown TypeError or ValueError.

Written by the indexing model from the issue text.