py3: have bytes version of os.getenv
os.getenv() on python 3 deals with unicodes. If we want to pass bytes. we have
os.getenvb() which deals with bytes. This patch adds up a pycompat.osgetenv
which deals with bytes on both python 2 and 3.
http://example.com/no/anchor, branches: (None, [])
http://example.com/an/anchor, branches: ('foo', [])
http://example.com/no/anchor/branches, branches: (None, ['foo'])
http://example.com/an/anchor/branches, branches: ('bar', ['foo'])
http://example.com/an/anchor/branches-None, branches: ('foo', [])
http://example.com/, branches: (None, [])
http://example.com/, branches: (None, [])
http://example.com/, branches: ('foo', [])