Don't suggest installing types-pycrypto (#10603)
It doesn't exist, since it was removed from typeshed in October (python/typeshed#4606). Similarly, pynamodb was removed in January.
I haven't tracked down what happened to pyre-extensions in typeshed, but https://pypi.org/project/types-pyre-extensions/ doesn't exist and in any case mypy users presumably don't need it.
