Update install command to the latest

[vinitkumar]

mypy-lang has moved to mypy now, so in order to install the latest mypy `pip install mypy is needed, not mypy-lang.

More details here:

• https://github.com/python/mypy/commit/339ba96d1c36894577db072a0f90656fea83a05a#diff-04c6e90faac2675aa89e2176d2eec7d8

[PiDelport]

I was about to submit this same PR. :)