scripts: modify shebang to use "/usr/bin/env python"

Before this patch, executing directly one of these scripts, for example
`./plot_hystogram.py` in a unix-like environment meant that the default
system-level python would be used, regardless of an eventual activated
virtualenv.

This was due to the "#!/usr/bin/python" shebang.
Changing it to "/usr/bin/env python" is a fairly standard practice, keeps intact
the compatiblity with the system level python, and allows a user to run in a
virtualenv if he wants.

scripts/plot_parametrized.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 
 """This program shows parametrized `hyperfine` benchmark results as an
 errorbar plot."""