Fix build with Python 3 scons.

Patch taken from Arch Linux:
https://github.com/archlinux/svntogit-packages/blob/packages/serf/trunk/scons-python3.patch

--- serf-1.3.9/SConstruct.orig	2019-07-26 17:49:30.910189251 +0000
+++ serf-1.3.9/SConstruct	2019-07-26 17:49:54.073821735 +0000
@@ -163,9 +163,9 @@
               suffix='.def', src_suffix='.h')
   })
 
-match = re.search('SERF_MAJOR_VERSION ([0-9]+).*'
-                  'SERF_MINOR_VERSION ([0-9]+).*'
-                  'SERF_PATCH_VERSION ([0-9]+)',
+match = re.search(b'SERF_MAJOR_VERSION ([0-9]+).*'
+                  b'SERF_MINOR_VERSION ([0-9]+).*'
+                  b'SERF_PATCH_VERSION ([0-9]+)',
                   env.File('serf.h').get_contents(),
                   re.DOTALL)
 MAJOR, MINOR, PATCH = [int(x) for x in match.groups()]
@@ -183,7 +183,7 @@
 
 unknown = opts.UnknownVariables()
 if unknown:
-  print 'Warning: Used unknown variables:', ', '.join(unknown.keys())
+  print ('Warning: Used unknown variables:', ', '.join(unknown.keys()))
 
 apr = str(env['APR'])
 apu = str(env['APU'])
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<div class='path'>path: <a href='/guix/log/?id=2c5f6787fd50553364922766af8805aff71105e3'>root</a>/<a href='/guix/log/tests?id=2c5f6787fd50553364922766af8805aff71105e3'>tests</a>/<a href='/guix/log/tests/substitute.scm?id=2c5f6787fd50553364922766af8805aff71105e3'>substitute.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/substitute.scm?id=2c5f6787fd50553364922766af8805aff71105e3&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2023-12-04 22:26:36 +0100'>2023-12-04</span></td><td><a href='/guix/commit/tests/substitute.scm?id=06b9c1260c72935806957bf302c40c1db6101a63'>substitute: Do not exit when failing to find a nar.</a><span class='msg-avail'>...<span class='msg-tooltip'>Fixes &lt;https://issues.guix.gnu.org/67575&gt;.

* guix/scripts/substitute.scm (process-substitution/fallback): Use
‘report-error’ instead of ‘leave’.  Write status line to PORT.
* tests/substitute.scm ("substitute, narinfo is available but nar is
missing"): Adjust accordingly.

Change-Id: Ic7297dbd563c007111ec2167c8d52505a07d4822
</span></span></td><td>Ludovic Courtès</td></tr>