fix mypy requirement at 0.910, for compatibility

[jepler]

This should fix the doc build, which currently fails

ulab/numpy/__init__.pyi:51: error: Cannot assign multiple types to name "_float" without an explicit "Type[...]" annotation
ulab/numpy/__init__.pyi:54: error: Cannot assign multiple types to name "_bool" without an explicit "Type[...]" annotation

since mypy 0.920 was released.

After

• Change how we alias the float and bool types for mypy v923z/micropython-ulab#461

or another fix is merged and our submodule ref is updated, we can un-pin mypy. However, as we may not want to change the version of ulab in 7.1.x right now, this workaround may be preferable.

[jepler]

Looks like we'll want to take #5740 instead.