Because 'bin/dotbot' is a hybrid sh/Python script that finds the
appropriate Python binary and then runs, the test code should not invoke
it as an sh script when sh does not source the pyenv initialization
script.
The fix implemented in this patch is to directly run 'bin/dotbot' as a
Python script using 'python', which when called from bash, will run with
the correct Python version (because bash is set up to source the pyenv
initialization script).
