Flake8 type checking (#2730)

CONTRIBUTING.md

@@ -52,6 +52,7 @@ below:
  - Mel Hall (Met Office, UK)
  - Ronnie Dutta (Met Office, UK)
  - Roddy Sharp (Met Office UK)
+ - Mark Dawson (Met Office UK)
 <!-- end-shortlog -->
 
 (All contributors are identifiable with email addresses in the version control

metomi/rosie/tests/test_suite_id.py

@@ -15,12 +15,14 @@
 # along with Rose. If not, see <http://www.gnu.org/licenses/>.
 
 import json
-from pathlib import Path
-from typing import Optional
+from typing import TYPE_CHECKING, Optional
 import pytest
 
 from metomi.rosie.suite_id import SuiteId
 
+if TYPE_CHECKING:
+    from pathlib import Path
+
 
 @pytest.mark.parametrize(
     'vcs_info, expected',
@@ -42,7 +44,7 @@
     ]
 )
 def test_parse_cylc_vc_file(
-    vcs_info: dict, expected: Optional[str], tmp_path: Path
+    vcs_info: dict, expected: Optional[str], tmp_path: 'Path'
 ):
     vcs_file = tmp_path / 'gimli.json'
     vcs_file.write_text(json.dumps(vcs_info))

mypy.ini

@@ -13,3 +13,7 @@ allow_redefinition = True
 
 strict_equality = True
 show_error_codes = True
+
+# Suppress the following messages:
+# By default the bodies of untyped functions are not checked, consider using --check-untyped-defs
+disable_error_code = annotation-unchecked

setup.cfg

@@ -81,6 +81,7 @@ rosa =
 tests =
     aiosmtpd
     flake8>=4.0.0
+    flake8-type-checking; python_version > "3.7"
     mypy>=0.800
     pytest
     types-aiofiles