Make Python 3.8 the default version

We promise compatibility with Python 3.8 and thus should use it as default.
Otherwise we might use modern Python features by accident and not realize
it until the integration tests run.

MODULE.bazel

@@ -46,13 +46,13 @@ python = use_extension(
     dev_dependency = True,
 )
 python.toolchain(
+    is_default = True,
     python_version = "3.8",
 )
 python.toolchain(
     python_version = "3.9",
 )
 python.toolchain(
-    is_default = True,
     python_version = "3.10",
 )
 python.toolchain(
@@ -69,7 +69,7 @@ pip_dev = use_extension(
 )
 pip_dev.parse(
     hub_name = "dwyu_mypy_deps",
-    python_version = "3.10",
+    python_version = "3.8",
     requirements_lock = "//third_party:mypy_requirements.txt",
 )
 use_repo(pip_dev, "dwyu_mypy_deps")

third_party/mypy_requirements.txt

@@ -1,5 +1,5 @@
 #
-# This file is autogenerated by pip-compile with Python 3.10
+# This file is autogenerated by pip-compile with Python 3.8
 # by the following command:
 #
 #    bazel run //third_party:mypy_requirements.update