run_mypy: add option to run once for each supported version of python

This allows verifying that meson is type-safe under older versions of
Python, which it currently is. Different versions of Python sometimes
have different supported types for an API.

Verify this in CI.

(We flush output to ensure CI prints lines in the right order.)

.github/workflows/lint.yml

@@ -47,4 +47,6 @@ jobs:
       with:
         python-version: '3.x'
     - run: python -m pip install mypy types-PyYAML
-    - run: python run_mypy.py
+    - run: python run_mypy.py --allver
+      env:
+        PYTHONUNBUFFERED: 1

run_mypy.py

@@ -104,6 +104,7 @@ def main() -> int:
     parser.add_argument('-q', '--quiet', action='store_true', help='do not print informational messages')
     parser.add_argument('-p', '--pretty', action='store_true', help='pretty print mypy errors')
     parser.add_argument('-C', '--clear', action='store_true', help='clear the terminal before running mypy')
+    parser.add_argument('--allver', action='store_true', help='Check all supported versions of python')
 
     opts, args = parser.parse_known_args()
     if opts.pretty:
@@ -129,8 +130,15 @@ def main() -> int:
         command = [opts.mypy] if opts.mypy else [sys.executable, '-m', 'mypy']
         if not opts.quiet:
             print('Running mypy (this can take some time) ...')
-        p = subprocess.run(command + args + to_check, cwd=root)
+        retcode = subprocess.run(command + args + to_check, cwd=root).returncode
-        return p.returncode
+        if opts.allver and retcode == 0:
+            for minor in range(7, sys.version_info[1]):
+                if not opts.quiet:
+                    print(f'Checking mypy with python version: 3.{minor}')
+                p = subprocess.run(command + args + to_check + [f'--python-version=3.{minor}'], cwd=root)
+                if p.returncode != 0:
+                    retcode = p.returncode
+        return retcode
     else:
         if not opts.quiet:
             print('nothing to do...')