projects/memtest: Fix the python shebang of the test scripts

If we're working in a virtualenv we wnat to use whatever python
is in the PATH and not a hardcoded one.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>

projects/memtest/sw/memtest-hyperram.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 import sys
 

projects/memtest/sw/memtest-spi.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 import binascii
 import random

projects/memtest/sw/memtest.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 import binascii
 import random