typing-extensions release 4.0.1

prepare release 4.0.0 (python#941)

prepare release 3.10.0.2 (python#873)

prepare release 3.10.0.1 (python#863)

prepare release 3.10.0.0 (python#805)

Revert last two changes; bump version to 3.7.4.3

This should address issues due to projects including 'typing' in their
requirements.txt file without a version constraint on Python (or
pinning to 3.7.4.1).

Bump version to 3.7.4.2

Bump version to 3.7.4.1 (python#670)

Bump version to 3.7.4 (python#651)

Bump version to 3.6.6 (python#581)