CI: Build native libraries for mrhlpr and mrtest via mypyc

Mostly for fun. Not sure if there's any real practical benefit. It might be useful for pmbootstrap later, but adopting it to be compatible with mypyc would likely be more work, so I'm using this as a playground to learn how it works.

.ci/native-build.sh

+#!/bin/sh -e
+# Description: Build native libraries of mrhlpr and mrtest using mypyc
+# https://postmarketos.org/pmb-ci
+
+if [ "$(id -u)" = 0 ]; then
+	set -x
+	apk -q add gcc py3-argcomplete py3-gitlab py3-pip python3-dev
+	exec su "${TESTUSER:-build}" -c "sh -e $0"
+fi
+
+set -x
+
+pip install --break-system-packages --no-warn-script-location mypy
+python -m mypyc mrtest mrhlpr
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