1.15.1 broke support for Optional type hinting schema generation.

• Support for Optional was added in Add support for custom and collection type hint classes #272
• I believe that support was broken in Fix unicode attribute error if typing backport installed on python2.7 #363 or 69b628a (part of 1.15.1).

Python versions on which the problem as been reproduced:

• CPython 3.7.3
• CPython 3.6.8
• PyPy 7.1.1 (3.6 compatible)

To reproduce, create a Serializer like:

from typing import Optional

class MySerializer(Serializer):
    x = SerializerMethodField()

    def get_x(self, instance) -> Optional[int]:
        pass

In 1.15.0, this is properly detected in the generated schema as an integer.
In 1.16.0 (I couldn't get 1.15.1 to run properly), it is incorrectly detected as a string.