python3: do not use hg when available

python3: do not use hg when available

During the execution of its configure script, Python 3 tries to find an
available "hg" installation, and if available, will try to use it to get
information from the version control system. To do this, it tries to
communicate over the network, potentially over ports that are blocked,
causing the build to halt. This was reported by a user as part of bug
7802.

To solve this, we simply make the Python script use /bin/false as the
"hg" program.

Fixes bug #7802 for the python3 package.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>