[CI] The latest mypy fails on current master

[dchigarev]

Mypy recently released a new 0.990 version that appears to be failing due to its internal error on modin's master.

$ pip install mypy==0.990
$ mypy --config-file mypy.ini --show-traceback
/localdisk/dchigare/repos/modin/modin/experimental/pandas/numpy_wrap.py:118: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 0.990
Traceback (most recent call last):
AssertionError: Error checking function redefinition FuncDef:118

The issue is already reported to mypy python/mypy#14027 but there's yet neither a fix nor an upcoming release to resolve it. So for now we should pin mypy to the lower version in order to make our CI work.