fpga: Replace references to python -> python3

In several cases, we call `python` explicitly. On many modern operating
systems (e.g., Ubuntu 22.04), this command no longer exists, and if it
does, it's often an alias to python3.

Because the python3 executable has been around for so long, simply
using that directly (instead of, e.g., autodetecting the OS preferences)
is a simple fix that immediately allows building bitfiles, e.g. for the
B2xx series, on more recent OSes.


Original-commit: 356cd172bb5ae0d043825bd1f4dffbd6646ec3d2

lib/io_port2/create-lvbitx.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/python3
 """
 Generate lvbitx from template and bitfile
 """