feat: improve python trace user experience.

[yihong0618]

for now, for python tracing work, we need the following steps.

1. chmod +x xxx.py
2. make sure we have shebang in front of python file

but in 2020s most python scripts do not executable and some of them do not have shebang,
if we want to trace it, we must make sure the two steps done.

if we can just run

# like uftrace a.out would be better 
uftrace xxx.py # this support all files ends with .py  

that would be better.
I would like to give a simple method and open one PR for that.

[namhyung]

The basic idea is to trace exactly what users do. Non-executable python script means it needs a python interpreter so I didn't support tracing the script directly.