support/scripts/check-uniq-files: add indices in format string

Using {} in format strings is only supported in sufficiently recent
Python versions. Python 2.6 doesn't support this, and only format
strings with numbered arguments: {0}, {1}, etc.

Python 2.7:

$ python -c 'print("foo {}".format(12))'
foo 12
$ python -c 'print("foo {0}".format(12))'
foo 12

Python 2.6:

$ python -c 'print("foo {}".format(12))'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ValueError: zero length field name in format
$ python -c 'print("foo {0}".format(12))'
foo 12

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>