Use actual pyp interpreter in shebang by default

[Bobronium]

Current version:

~
$ pyp "'pyp is running on ' + sys.executable" --explain > explained.py

~
$ perl explained.py
Traceback (most recent call last):
  File "/Users/bobronium/explained.py", line 3, in <module>
    from pyp import pypprint
ModuleNotFoundError: No module named 'pyp'

~
$ cat explained.py
#!/usr/bin/env python3
import sys
from pyp import pypprint
assert sys.stdin.isatty() or not sys.stdin.read(), "The command doesn't process input, but input is present"
output = 'pyp is running on ' + sys.executable
if output is not None:
    pypprint(output)

With these changes:

~
$ pyp "'pyp is running on ' + sys.executable" --explain > explained.py

~
$ perl explained.py
pyp is running on /Users/bobronium/.local/pipx/venvs/pypyp/bin/python

~
$ cat explained.py
#!/Users/bobronium/.local/pipx/venvs/pypyp/bin/python
import sys
from pyp import pypprint
assert sys.stdin.isatty() or not sys.stdin.read(), "The command doesn't process input, but input is present"
output = 'pyp is running on ' + sys.executable
if output is not None:
    pypprint(output)

[Bobronium]

Sorry, I've been too lazy to clone it and test it properly. Just patched my installation to check that it's working as intended.

I'll update the tests soon and will request review when it'll be ready for one, in the meanwhile this should be a draft really.