util,python: Fix Pre-commit hooks to ignore non-source files

util,python: Fix Pre-commit hooks to ignore non-source files

Previously if binary blobs were modified the pre-commit hook attempted
to run style-checks on the binary, causing an error when attempting to
decode to utf-8. This commit runs a check on each file to ensure it has
a valid source-code extension prior to running style checks. If a file
does not have a valid extension style checks are not run.

Change-Id: Id1263cac0d6c190ad1a3d67720b3f373e0e42234
Issue-on: https://gem5.atlassian.net/browse/GEM5-903
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39795
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>