readlink not supported on Windows and Python <3.2

[mottosso]

Used by rez cp --follow-symlinks.

Rez uses readlink to follow symlinks during copy. It's supported cross-platform since Python 3.2.

• https://docs.python.org/3.5/library/os.html#os.readlink

https://github.com/mottosso/bleeding-rez/blob/21b92cc4a762f6445b8eae96a66b4aae6c804b1e/src/rez/utils/filesystem.py#L214-L218