Remove redundant mypy config 'incremental'

The mypy config 'incremental` has defaulted as true since version 0.590.
Can drop the local override. For details, see:

6b13652a46

setup.cfg

@@ -47,7 +47,6 @@ show_column_numbers = True
 show_error_context = True
 ignore_missing_imports = True
 follow_imports = skip
-incremental = True
 check_untyped_defs = True
 warn_unused_ignores = True
 strict_optional = False

setup.py

@@ -43,7 +43,7 @@ extras_require = {
         'html5lib',
         'flake8>=3.5.0',
         'flake8-import-order',
-        'mypy>=0.470',
+        'mypy>=0.590',
         'docutils-stubs',
     ],
 }