Read from a file-like object

[IamJeffG]

This PR extends GuanoPy to allow reading from any file-like object (one that implements methods seek, read, tell), not just via filename.

This is important when the very large WAV file exists over a network and we want to extract GUANO metadata without downloading the entire sound data.

What I changed

• GuanoPy.__init__ arg switched from filename to file:

  • If file is a string, opens the file by that name,
  • otherwise if file is set, treat it as a file-like object (one that implements methods seek, read, tell).

• Add filename property that returns a string only if this GuanoPy object references a file by its filename. This is settable if you want to write a "new" file.

• Places that GuanoPy actually has to open the file (_load and write methods), check if the self.filename property is set (i.e. we have a string); if so, open it as usual, otherwise simply return the file-like object from a contextlib.nullcontext()

• New test case to read a from file-like object: an already open file handle.