Support for files that specify their encoding

[kmulka-bloomberg]

I ran into the same issue many had here: https://github.com/bndr/pipreqs/issues/214

Python source code does have a standard way of specifying the encoding of a file. I suggest pipreqs uses this if available to detect the encoding of the file.

PEP-263: Defining Python Source Code Encodings https://peps.python.org/pep-0263/