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'])
/'>summary</a><a href='/guix/refs/?id=8771fc1715b400ead16f97683c67ad336b485750'>refs</a><a class='active' href='/guix/log/gnu/ci.scm'>log</a><a href='/guix/tree/gnu/ci.scm?id=8771fc1715b400ead16f97683c67ad336b485750'>tree</a><a href='/guix/commit/gnu/ci.scm?id=8771fc1715b400ead16f97683c67ad336b485750'>commit</a><a href='/guix/diff/gnu/ci.scm?id=8771fc1715b400ead16f97683c67ad336b485750'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/ci.scm'>
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<div class='path'>path: <a href='/guix/log/?id=8771fc1715b400ead16f97683c67ad336b485750'>root</a>/<a href='/guix/log/gnu?id=8771fc1715b400ead16f97683c67ad336b485750'>gnu</a>/<a href='/guix/log/gnu/ci.scm?id=8771fc1715b400ead16f97683c67ad336b485750'>ci.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/gnu/ci.scm?id=8771fc1715b400ead16f97683c67ad336b485750&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2019-01-21 10:19:31 +0100'>2019-01-21</span></td><td><a href='/guix/commit/gnu/ci.scm?id=c680a7daa5e143dd37d1d045805e073497c591be'>ci: Skip system tests on armhf-linux.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/ci.scm (system-test-jobs): Return the empty list when SYSTEM is
"armhf-linux".
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-01-20 01:42:11 +0100'>2019-01-20</span></td><td><a href='/guix/commit/gnu/ci.scm?id=7e6d8d366a61f951936ed83371877ce006f679f6'>ci: Use a valid 'current-guix'.</a><span class='msg-avail'>...<span class='msg-tooltip'>This fixes a regression introduced in
b5f8c2c88543158e8aca76aa98f9009f6b9e743a whereby 'current-guix' (needed
by some of the system tests) would fail to build.
Reported by Ricardo Wurmus &lt;rekado@elephly.net&gt;.

It also speeds up compilation of 'current-guix' since the channel
instance is already compiled or can be built quickly compared to the
default 'current-guix'.

* gnu/packages/package-management.scm (current-guix-package): New
variable.
(current-guix): Honor it.
* gnu/ci.scm (channel-build-system): New variable.
(channel-instances-&gt;derivation): New procedure.
(system-test-jobs): Add #:source and #:commit parameters.
Define 'instance' and parameterize CURRENT-GUIX-PACKAGE.
(hydra-jobs)[checkout, commit, source]: New variables.
Pass #:source and #:commit to 'system-test-jobs'.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-01-06 21:41:07 +0100'>2019-01-06</span></td><td><a href='/guix/commit/gnu/ci.scm?id=b5f8c2c88543158e8aca76aa98f9009f6b9e743a'>hydra: Compute jobs in an inferior.</a><span class='msg-avail'>...<span class='msg-tooltip'>Previously we would rely on auto-compilation of all the Guix modules.
The complete evaluation would take ~15mn on berlin.guixsd.org and
require lots of RAM.  This approach should be faster since potentially
only part of the modules are rebuilt.  Furthermore, as a side-effect, it
builds the derivations that 'guix pull' uses.

* build-aux/hydra/gnu-system.scm: Remove 'eval-when' form.
(hydra-jobs): New procedure.
* gnu/ci.scm (package-&gt;alist, qemu-jobs, system-test-jobs)
(tarball-jobs): Return strings for the 'license' field.
* guix/self.scm (compiled-guix)[*cli-modules*]: Add (gnu ci).
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-01-06 21:41:07 +0100'>2019-01-06</span></td><td><a href='/guix/commit/gnu/ci.scm?id=59fb5c1cdb6fa5d6d1dbeca58e33f4a01a7d98f8'>hydra: Move job definitions to (gnu ci).</a><span class='msg-avail'>...<span class='msg-tooltip'>* build-aux/hydra/gnu-system.scm: Move code to...
* gnu/ci.scm: ... here.  New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
</span></span></td><td>Ludovic Courtès</td></tr>