Allow specifying encodings

[flying-sheep]

This e.g. allows using future-fstrings, which I’d like to use in flying-sheep/get_version#3

You can check that this works via:

#!/usr/bin/env python3.5
import ast
st = ast.parse(b"# coding: future-fstrings\nprint(f'hi {ast}')")
exec(compile(st, filename="", mode="exec"))
# hi <module 'ast' from '/usr/lib64/python3.5/ast.py'>

[takluyver]

The Path.read_bytes() method was new in 3.5, so there's a test failure on 3.4. However, I think we can move the minimum supported version to 3.5 at this point.

[flying-sheep]

well, or we can open the file in binary mode. as you wish.

[takluyver]

I'm OK with dropping 3.4, but I don't feel a strong need to, so since you've made it work with 3.4...

[flying-sheep]

Great, thank you!