Allow user to specify whether TYPE_CHECKING should be true or not

[lihu-zhong]

A recent commit 86d0834 sets TYPE_CHECKING to True unconditionally, which breaks a workaround I had been using for a bug in typing. Essentially, it works like this:

if TYPE_CHECKING:
    class MyType:
        """This is the actual class I need, but it falls over at runtime due to a bug"""
        ...
else:
    class MyType:
        """This is a dummy class"""
        ...

You can see the full thread here: python/typing#561 (comment)

Please make this variable user-configurable; I can't build my docs with this change. Thank you!

[agronholm]

I'm going to have to find out how to add configuration options to a Sphinx plugin.

[agronholm]

Fixed via 2afccd3, v1.5.0 released.

[lihu-zhong]

Thank you for the fast turn around!