linting: drop installing mypy via pip, pull from Debian unstable

linting

@@ -6,11 +6,10 @@ RUN apt-get update \
     gir1.2-gdkpixbuf-2.0 \
     gir1.2-poppler-0.18 \
     gir1.2-rsvg-2.0 \
+    mypy \
     pyflakes3 \
     pylint3 \
     python3-bandit \
     python3-gi-cairo \
     python3-mutagen \
-    python3-pip \
-&&  rm -rf /var/cache/apt/* /var/lib/apt/lists/* \
-&&  pip3 install mypy
+&&  rm -rf /var/cache/apt/* /var/lib/apt/lists/*