Eventually upgrade mypy

[appleby]

In #1119 @erichulburd noticed that make typecheck was failing with mypy 0.750 and so pinned our mypy dependency to 0.740 as a workaround.

The type errors on mypy 0.750 stem from the definition of ExpressionDesignator in quilatom.py, which is a nested Union type like so:

ExpressionValueDesignator = Union[int, float, complex]
ExpressionDesignator = Union['Expression', ExpressionValueDesignator]

Long story short, mypy 0.750 doesn't like the nested ExpressionValueDesignator.

As a workaround, flattening the Union like so resolves the issue:

ExpressionValueDesignator = Union[int, float, complex]
-ExpressionDesignator = Union['Expression', ExpressionValueDesignator]
+ExpressionDesignator = Union['Expression', int, float, complex]

I've opened a mypy issue upstream.

For now we should just keep mypy pinned to 0.740, but when/if the above mypy issue is addressed, we me might consider upgrading, or else apply the above patch if the mypy issue is resolved as "wont fix".

[appleby]

The upstream mypy issue was fixed in https://github.com/python/mypy/pull/8146.

Should be safe to upgrade to the whatever the next release is after 0.750.

[TH3CHARLie]

I think although the issue is fixed in python/mypy#8146, it will be not in the release until v0.770(the most recent release is v0.761 where this PR has not merged yet), so you may still stay with v0.740 for maybe a month(or shorter) till v0.770 comes out.

[appleby]

Good to know @TH3CHARLie thanks for the heads up (and for fixing the bug :-)

[rht]

Mypy 0.770 was out 22 days ago: https://github.com/python/mypy/releases/tag/v0.770.