sslutil: fsencode path returned by certifi (issue6132) stable

sslutil: fsencode path returned by certifi (issue6132)

By inspection, this is the only codepath that could be returning a
string instead of a bytes on Python 3.

--- a/mercurial/sslutil.py	Mon May 06 22:10:34 2019 -0400
+++ b/mercurial/sslutil.py	Wed May 08 16:09:50 2019 -0400
@@ -722,7 +722,7 @@
         certs = certifi.where()
         if os.path.exists(certs):
             ui.debug('using ca certificates from certifi\n')
-            return certs
+            return pycompat.fsencode(certs)
     except (ImportError, AttributeError):
         pass