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.

mercurial/pycompat.py

@@ -45,6 +45,7 b' if ispy3:'
     ospathsep = os.pathsep.encode('ascii')
     ossep = os.sep.encode('ascii')
     osaltsep = os.altsep
+    osgetenv = os.getenvb
     if osaltsep:
         osaltsep = osaltsep.encode('ascii')
     # os.getcwd() on Python 3 returns string, but it has os.getcwdb() which
@@ -156,6 +157,7 b' else:'
     sysargv = sys.argv
     sysplatform = sys.platform
     getcwd = os.getcwd
+    osgetenv = os.getenv
 
 stringio = io.StringIO
 empty = _queue.Empty