contrib: header-tools scripts updated to python3

The scripts in contrib/header-tools/ are incompatible with python3.
This updates them to use python3.

contrib/header-tools/ChangeLog:

	* count-headers: Adapt to Python 3.
	* gcc-order-headers: Likewise.
	* graph-header-logs: Likewise.
	* graph-include-web: Likewise.
	* headerutils.py: Likewise.
	* included-by: Likewise.
	* reduce-headers: Likewise.
	* replace-header: Likewise.
	* show-headers: Likewise.

Signed-off-by: Sundeep KOKKONDA <sundeep.kokkonda@windriver.com>