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/ni_hw_models.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Copyright 2016 Ettus Research
 #
@@ -16,8 +16,8 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #
 
-import rfnocsim
 import math
+import rfnocsim
 
 class UsrpX310(rfnocsim.SimComp):
     # Hardware specific constants