Install python3 in Mint

Signed-off-by: Alexey Neyman <stilor@att.net>

testing/docker/mint19-amd64/Dockerfile

@@ -5,7 +5,7 @@ RUN groupadd -g $CTNG_GID ctng
 RUN useradd -d /home/ctng -m -g $CTNG_GID -u $CTNG_UID -s /bin/bash ctng
 RUN apt-get update
 RUN apt-get install -y gcc gperf bison flex texinfo help2man make libncurses5-dev \
-    python-dev autoconf automake libtool libtool-bin gawk wget
+    python3-dev autoconf automake libtool libtool-bin gawk wget
 RUN wget -O /sbin/dumb-init https://github.com/Yelp/dumb-init/releases/download/v1.2.1/dumb-init_1.2.1_amd64
 RUN chmod a+x /sbin/dumb-init
 RUN echo 'export PATH=/opt/ctng/bin:$PATH' >> /etc/profile