setup: use sysstr() on process output stable

setup: use sysstr() on process output

Otherwise we get a str-bytes mismatch on Python 3 if
an error occurs.

Differential Revision: https://phab.mercurial-scm.org/D8469

--- a/setup.py	Sat Mar 28 08:18:11 2020 -0700
+++ b/setup.py	Sun Apr 19 17:26:52 2020 -0700
@@ -890,7 +890,8 @@
             )
             if res:
                 raise SystemExit(
-                    'error running gendoc.py: %s' % '\n'.join([out, err])
+                    'error running gendoc.py: %s'
+                    % '\n'.join([sysstr(out), sysstr(err)])
                 )
 
             with open(txt, 'wb') as fh:
@@ -907,7 +908,8 @@
             )
             if res:
                 raise SystemExit(
-                    'error running gendoc: %s' % '\n'.join([out, err])
+                    'error running gendoc: %s'
+                    % '\n'.join([sysstr(out), sysstr(err)])
                 )
 
             with open(gendoc, 'wb') as fh:
@@ -932,7 +934,8 @@
             )
             if res:
                 raise SystemExit(
-                    'error running runrst: %s' % '\n'.join([out, err])
+                    'error running runrst: %s'
+                    % '\n'.join([sysstr(out), sysstr(err)])
                 )
 
             normalizecrlf('doc/%s' % root)
@@ -957,7 +960,8 @@
             )
             if res:
                 raise SystemExit(
-                    'error running runrst: %s' % '\n'.join([out, err])
+                    'error running runrst: %s'
+                    % '\n'.join([sysstr(out), sysstr(err)])
                 )
 
             normalizecrlf('doc/%s.html' % root)