Use Python 3.8 in runtests.py

We only support running mypy using 3.8+ up, since mypy 1.5 dropped
support for running with Python 3.7.

Closes: python#10599

Author: srittau

scripts/runtests.py

@@ -28,7 +28,7 @@ def colored(text: str, color: str | None = None, **kwargs: Any) -> str:  # type:
 _FAILED = colored("Failed", "red")
 # We're using the oldest supported version because it's the most likely to produce errors
 # due to unsupported syntax, feature, or bug in a tool.
-_PYTHON_VERSION = "3.7"
+_PYTHON_VERSION = "3.8"
 
 
 def _parse_jsonc(json_text: str) -> str: