CI: Install mypy from pip instead of Alpine (MR 2425)

Alpine stable's mypy is currently outdated and has bugs in its
handling of overloads. As such, just use mypy from pip so we don't have
to worry about working aroud bugs in old mypy versions.

[ci:skip-build]: already built successfully in CI

.ci/mypy.sh

@@ -4,10 +4,11 @@
 
 if [ "$(id -u)" = 0 ]; then
 	set -x
-	apk -q add py3-argcomplete py3-mypy
+	apk -q add py3-argcomplete py3-pip
 	exec su "${TESTUSER:-build}" -c "sh -e $0"
 fi
 
 set -x
 
-mypy --color-output pmbootstrap.py
+pip install --break-system-packages --no-warn-script-location mypy
+python -m mypy --color-output pmbootstrap.py