Bump typing-extensions from 3.7.4.2 to 3.7.4.3 (#97)

Bumps [typing-extensions](https://github.com/python/typing) from 3.7.4.2 to 3.7.4.3.
- [Release notes](https://github.com/python/typing/releases)
- [Commits](python/typing@3.7.4.2...3.7.4.3)

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