Don't open a window when supported

[ghost]

The console window which opens every time when accessing ffmpeg sucks. This PR will stop this behaviour when it's supported by the Python version (introduced in 3.7)

Successor of #88.

[sampsyo]

Cool; thanks! It looks like the existence test is necessary even on 3.7, because the constant is only available on Windows. In fact, it looks like the common thing to do on GitHub is to hard-code the value of this constant: https://github.com/search?q=subprocess.CREATE_NO_WINDOW&type=Code

Perhaps we should just do that?

Also, instead of modifying the popen_multiple function to do this dynamically, which has the unfortunate consequence of preventing over creationflags from being added in the future, what do you think about doing this at the top level (module scope)?

if hasattr(subprocess, 'CREATE_NO_WINDOW'):
    PROC_FLAGS = subprocess.CREATE_NO_WINDOW
else:
    PROC_FLAGS = 0

Then, pass creationflags=PROC_FLAGS in both calls to popen_multiple.

[ghost]

Assuming we'd hardcode it, wouldn't we have to check to see if we're on Windows? (Look in the subprocess code l45, l185) And we can remove the version condition for that, right?

if sys.platform == "win32":
    PROC_FLAGS = 0x08000000
else:
    PROC_FLAGS = 0

[sampsyo]

Yeah, that seems about right!

[sampsyo]

Looks perfect; thank you!! :sparkles: :rocket: