automation: drop support for Python 2.7 in Linux environment
We stop installing Python 2.7 via pyenv. We stop installing the system
Python 2 packages. We delete support for running tests on Python 2.7.
Differential Revision: https://phab.mercurial-scm.org/D12262
Hello, Foobar
[b'Hello, Fo, =;:<o', b'Hello, Bar']