add missing plugin entrypoint, add plugins.ini mypy config with plugins specified

mypy_django_plugin/plugins/postgres_fields.py

@@ -9,3 +9,7 @@ class PostgresFieldsPlugin(Plugin):
     def get_base_class_hook(self, fullname: str
                             ):
         return determine_type_of_array_field
+
+
+def plugin(version):
+    return PostgresFieldsPlugin

test/plugins.ini

@@ -0,0 +1,2 @@
+[mypy]
+plugins = mypy_django_plugin.plugins.postgres_fields

test/testdjango.py

@@ -9,6 +9,7 @@ from mypy.test.helpers import assert_string_arrays_equal
 
 ROOT_DIR = Path(__file__).parent.parent
 TEST_DATA_DIR = ROOT_DIR / 'test' / 'test-data'
+MYPY_INI_PATH = ROOT_DIR / 'test' / 'plugins.ini'
 
 
 class DjangoTestSuite(DataSuite):
@@ -23,7 +24,8 @@ class DjangoTestSuite(DataSuite):
 
         mypy_cmdline = [
             '--show-traceback',
-            '--no-silence-site-packages'
+            '--no-silence-site-packages',
+            '--config-file={}'.format(MYPY_INI_PATH)
         ]
         mypy_cmdline.append('--python-version={}'.format('.'.join(map(str,
                                                                       sys.version_info[:2]))))