Bump max mypy version

[mschoettle]

I just updated mypy to 0.782 and got the following error:

ERROR: django-stubs 1.5.0 has requirement mypy<0.780,>=0.770, but you'll have mypy 0.782 which is incompatible.

So far I have not encountered any issues despite this error (it still installed it regardless).

Can the max version requirement be safely increased?

[kszmigiel]

@mschoettle Mypy version in requirements was already bumped, but we haven't released 1.6.0 yet
@sobolevn I think updating Mypy requirement to 0.782 should do no harm, I've opened a PR, all tests are passing