Refuse to run by default if python version is not 2

Refuse to run by default if python version is not 2

Exit with an error if the python major version isn't 2, since we're
working on support for py3, and we'll probably reach a point before
we're finished where bup doesn't immediately crash with py3, but might
well do very bad things to the repository.

Allow overriding the check by setting
BUP_ALLOW_UNEXPECTED_PYTHON_VERSION=true so people can still test py3
if they like.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>