bot: Use Mercurial with Python 3

[marco-c]

Fixes #332

[marco-c]

From ~104 MB to ~83,5 MB.

[La0]

It works

lao@excalibur:~ docker load -i Downloads/code-coverage-bot\(1\).tar 
965ae6d92530: Loading layer [==================================================>]  4.022kB/4.022kB
7bf898e6c3e0: Loading layer [==================================================>]   23.2kB/23.2kB
2244af031428: Loading layer [==================================================>]  12.88MB/12.88MB
64f80685524a: Loading layer [==================================================>]  3.667MB/3.667MB
12691c5e5835: Loading layer [==================================================>]  10.88MB/10.88MB
The image registry.hub.docker.com/mozilla/code-coverage:bot-dev already exists, renaming the old one with ID sha256:6aa64f7b9ccc8d327fb03440408044f31998898961ded6f982cd46813ce93d0c to empty string
Loaded image: registry.hub.docker.com/mozilla/code-coverage:bot-dev
Loaded image: registry.hub.docker.com/mozilla/code-coverage:bot-fbef2590182c0b6f20a2235351f5977fc0ed3524
lao@excalibur:~ docker run -it registry.hub.docker.com/mozilla/code-coverage:bot-dev bash
root@ef0950872fec:/# hg version
Mercurial Distributed SCM (version 5.2)
(see https://mercurial-scm.org for more information)

Copyright (C) 2005-2019 Matt Mackall and others
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
root@ef0950872fec:/# python2
bash: python2: command not found
root@ef0950872fec:/# python
Python 3.7.6 (default, Jan  3 2020, 23:35:31) 
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 

[marco-c]

robustcheckout doesn't work yet though, so we need to wait for it to be fixed before being able to merge this.

[La0]

😿 https://bugzilla.mozilla.org/show_bug.cgi?id=1604652

[marco-c]

The bug has been fixed!