Apply explicit_package_bases for mypy and unpin the version. Ref python/mypy#14057.

Author: jaraco

mypy.ini

@@ -1,2 +1,5 @@
 [mypy]
 ignore_missing_imports = True
+# required to support namespace packages
+# https://github.com/python/mypy/issues/14057
+explicit_package_bases = True

setup.cfg

@@ -40,8 +40,6 @@ testing =
 	pytest-mypy >= 0.9.1; \
 		# workaround for jaraco/skeleton#22
 		python_implementation != "PyPy"
-	# workaround for realpython/pytest-mypy#141
-	mypy < 0.990
 	pytest-enabler >= 1.3
 
 	# local