Do not hardcode path to Python3, because on e.g. OSX it can be anywhere.

test cases/common/61 custom target source output/generator.py

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

test cases/common/64 custom header generator/makeheader.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 # NOTE: this file does not have the executable bit set. This tests that
 # Meson can automatically parse shebang lines.

test cases/common/65 multiple generators/mygen.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 import sys, os
 
@@ -16,4 +16,4 @@ open(outhdr, 'w').write('int func%s();\n' % val)
 open(outsrc, 'w').write('''int func%s() {
     return 0;
 }
-''' % val)
+''' % val)