If `bin/pyang` is given a path to a YANG file which contains more than
one '@' character (e.g. if there's an '@' in a parent directory), the
`re_filename` regexp will detect the revision-date as being everything
from the first instances of the '@' char in the path, to the yang/tin
suffix(es).

This fails to match the expected 'YYYY-MM-DD' date format that the
revision-date in the filename should take and `pyang` fails.

This commit fixes `re_filename` regexp so that it disregards all
characters in the filepath before the last directory separator (based
on the value of `os.sep`) in the string.