packaging: update Ubuntu docker build dependencies to Python 3

Changeset 7574ccd87200f02e updated the Debian docker builds to Python 3. In doing
so, the Ubuntu docker-based builds broke, as the Dockerfile does not
include the new dependencies as specified in `contrib/packaging/debian/control`.

This commit changes the dependencies in the Ubuntu Dockerfile template
to their Python 3 equivalents, fixing `make docker-ubuntu-bionic`.

Differential Revision: https://phab.mercurial-scm.org/D7810

contrib/packaging/docker/ubuntu.template

@@ -1,16 +1,17 b''
 FROM ubuntu:%CODENAME%
 
 RUN groupadd -g 1000 build && \
     useradd -u 1000 -g 1000 -s /bin/bash -d /build -m build
 
 RUN apt-get update && apt-get install -y \
   build-essential \
   debhelper \
   devscripts \
   dh-python \
   less \
   python \
-  python-all-dev \
-  python-docutils \
+  python3-all \
+  python3-all-dev \
+  python3-docutils \
   unzip \
   zip