support/docker: use python3-flake8

support/scripts/pkg-stats now uses some Python 3.x only constructs
("async" and related keywords), so we must use the Python 3.x flake8.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 385c4da3dd10b6bfbaf23c04f48e7835ce5cc701)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/docker/Dockerfile

@@ -36,10 +36,10 @@ RUN apt-get install -y --no-install-recommends \
         libncurses5-dev \
         locales \
         mercurial \
-        python-flake8 \
         python-nose2 \
         python-pexpect \
         python3 \
+        python3-flake8 \
         python3-nose2 \
         python3-pexpect \
         qemu-system-arm \