can: amend recently added Python output feature

can: amend recently added Python output feature

Adjust nits in the previous commit which added Python output to the CAN
decoder. Rename the routine to .putpy() for consistency with other PDs.
Avoid the 'bytes' identifier as it ships with Python as a data type.
Rephrase the construction of the tuple for Python output, to become more
explicit and help reviewers. Partially "undo" the rtr rename, to reduce
diffs to the previous version (see diff HEAD~2 to see the essence). Fix
up a three space indentation issue (which was there before the addition
of Python output).