Move future statements at the top

[maneandrea]

Description:
Trying to stickytape a file containing a future statement results in an error:

SyntaxError: from __future__ imports must occur at the beginning of the file

Steps to reproduce:

1. Write file a.py

from __future__ import annotations

def f(x: str | int) -> str | int:
    return 2 * x

2. Run stickytape a.py > b.py
3. Run python b.py

Expected result:
Nothing

Actual result:

  File "b.py", line 37
    from __future__ import annotations
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
SyntaxError: from __future__ imports must occur at the beginning of the file

Possible fix:
Detect such statements, for instance with a regex pattern, and move them at the beginning of the file.

re.match('from __future__ import (.*)$', line)

[mwilliamson]

Adding future imports changes the meaning of the code, so changing them to apply to everything sounds fragile to me.

In general, I'd suggest using something like zipapp instead of this pile of hacks.

[maneandrea]

That's a good point. I did a PR anyway because I find it useful for my particular use case. If you are against it, feel free to close it.