Replace `py3_path`, `bytestring_path`, `cpu_count` implementations with builtins

[snejus]

• Replace py3_path by standard os.fsdecode
• Use native os.cpu_count
• Replace bytestring_path with os.fsencode

Description

These functions had been written before builtins achieving the same have been made available (in Python 3.2 and 3.4).

Now that they are available though, this PR updates the codebase to use builtin implementations rather than the custom ones.