macosx: fixing macOS version generation after db9d1dd01bf0

With the Python3 change, the string is now something like
version = b'4.6.2+848-88be288e8ac1'
where it was previously just:
version = '4.6.2+848-88be288e8ac1'

Differential Revision: https://phab.mercurial-scm.org/D3964

contrib/genosxversion.py

@@ -117,9 +117,9 b' def main(argv):'
         return
     with open(opts.versionfile) as f:
         for l in f:
-            if l.startswith('version = '):
+            if l.startswith('version = b'):
                 # version number is entire line minus the quotes
-                ver = l[len('version = ') + 1:-2]
+                ver = l[len('version = b') + 1:-2]
                 break
     if opts.paranoid:
         print(paranoidver(ver))