It already can be used for development, IDEs such as PyCharm perform quite effective live type checking based on type hints.
Note that PyCharm and mypy's semantics differ in some key ways:
https://youtrack.jetbrains.com/issue/PY-54411/Type-checkers-....
https://youtrack.jetbrains.com/issue/PY-24834/Support-strict...
But the type-checking could be a command line parameter, used only during development.