fpga: Remove Python2 support from build system

- 2to3 was used to convert the Python scripts, except where the tool
  choked and manual intervention was required
- All references to "python" where replaced with "python3"
- buffer() was replaced by memoryview()


Original-commit: ca68195b5d12c5410cfac8d459a0b0902c4c72c7

tools/utils/rfnoc-system-sim/README

@@ -1,6 +1,6 @@
 Dependencies:
-- python2
 - graphviz
-- python-graphviz
-- python-numpy
-- python-matplotlib
+- python3
+- python3-graphviz
+- python3-numpy
+- python3-matplotlib