test_mypy fails

[hynek]

Something changed around mypy and our tests are failing: https://github.com/python-attrs/attrs/runs/4030751250?check_suite_focus=true#step:5:57

cc @euresti

(sorry for terseness, I'm on the jump rn)

``` /home/runner/work/attrs/attrs/tests/test_mypy.yml:784: 59 E pytest_mypy_plugins.utils.TypecheckAssertionError: Invalid output: 60 E Actual: 61 E main:15: error: Cannot determine __init__ type from converter (diff) 62 E main:15: error: Argument "converter" has incompatible type "Callable[[], str]"; expected "Callable[[Any], Any]" (diff) 63 E main:16: error: Cannot determine __init__ type from converter (diff) 64 E main:16: error: Argument "converter" has incompatible type overloaded function; expected "Callable[[Any], Any]" (diff) 65 E ... 66 E Expected: 67 E main:15: error: Argument "converter" has incompatible type "Callable[[], str]"; expected "Callable[[Any], Any]" (diff) 68 E main:15: error: Cannot determine __init__ type from converter (diff) 69 E main:16: error: Argument "converter" has incompatible type overloaded function; expected "Callable[[Any], Any]" (diff) 70 E main:16: error: Cannot determine __init__ type from converter (diff) 71 E ... 72 E Alignment of first line difference: 73 E E: main:15: error: Argument "converter" has incompatible type "Callable[[],... 74 E A: main:15: error: Cannot determine __init__ type from converter... 75 E ^ 76 _____________________ testAttrsUsingBadConverterReprocess ______________________ 77 /home/runner/work/attrs/attrs/tests/test_mypy.yml:813: 78 E pytest_mypy_plugins.utils.TypecheckAssertionError: Invalid output: 79 E Actual: 80 E main:16: error: Cannot determine __init__ type from converter (diff) 81 E main:16: error: Argument "converter" has incompatible type "Callable[[], str]"; expected "Callable[[Any], Any]" (diff) 82 E main:17: error: Cannot determine __init__ type from converter (diff) 83 E main:17: error: Argument "converter" has incompatible type overloaded function; expected "Callable[[Any], Any]" (diff) 84 E ... 85 E Expected: 86 E main:16: error: Argument "converter" has incompatible type "Callable[[], str]"; expected "Callable[[Any], Any]" (diff) 87 E main:16: error: Cannot determine __init__ type from converter (diff) 88 E main:17: error: Argument "converter" has incompatible type overloaded function; expected "Callable[[Any], Any]" (diff) 89 E main:17: error: Cannot determine __init__ type from converter (diff) 90 E ... 91 E Alignment of first line difference: 92 E E: main:16: error: Argument "converter" has incompatible type "Callable[[],... 93 E A: main:16: error: Cannot determine __init__ type from converter... 94 E ^ ```

[euresti]

See #856