Release 4.2.0 planning

[JelleZijlstra]

I'd like to release typing-extensions 4.2.0 later this weekend. I'll merge python/typing#1139 and python/typing#1141 first.

Major features will include:

• Drop support for 3.6
• Add assert_type()
• Add get_overloads() and clear_overloads()
• dataclass_transform change from dataclass_transform: accept **kwargs, rename field_descriptors typing#1120

Let me know if you'd like to see any other changes.

[JelleZijlstra]

https://pypi.org/project/typing-extensions/4.2.0/#description is out!