Python typehints: merge unions of tuples of the same size

When inferring PCollection element types, merge Unions of identically-sized Tuples:
Union[Tuple[a, b], Tuple[c, d]] =\> Tuple[Union[a, c], Union[b, d]]

cc: [~robertwb]

Example:
```

def fn(element: Tuple[str, Any]):
....
p | Create([('a', None), ('b', None)] + [('c', 1), ('d',
3)]) | ParDo(fn)

```

This pipeline will fail with something like:
```

apache_beam.typehints.decorators.TypeCheckError: Type hint violation for 'ParDo': requires Tuple[str,
Any] but got Union[Tuple[str, NoneType], Tuple[str, int]] for element

```

In test form:
```

def test_union_of_tuple(self):
self.assertCompatible(
typehints.Tuple[str, typehints.Any],

typehints.Union[typehints.Tuple[str, type(None)],
typehints.Tuple[str,
int]])

```

Imported from Jira [BEAM-8464](https://issues.apache.org/jira/browse/BEAM-8464). Original Jira may contain additional context.
Reported by: udim.