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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<tr><td><span title='2021-01-04 11:51:48 +0100'>2021-01-04</span></td><td><a href='/guix/commit/gnu/machine?id=61d8bd56a4644a72b964b680456da4332eb1ef6d'>machine: ssh: Do not import the host (guix config), really.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-12-21 18:41:11 +0100'>2020-12-21</span></td><td><a href='/guix/commit/gnu/machine?id=f00e68ace070fd5240a4b5874e61c26f6e909b6c'>system: Allow separated /boot and encrypted root.</a><span class='msg-avail'>...</span></td><td>Miguel Ángel Arruga Vivas</td></tr>
<tr><td><span title='2020-11-09 23:27:09 +0100'>2020-11-09</span></td><td><a href='/guix/commit/gnu/machine?id=70ffa8af1e93ab8a92c4622745e9cb4a2782f3c8'>machine: ssh: Do not import the host (guix config).</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-11-01 02:28:38 +0100'>2020-11-01</span></td><td><a href='/guix/commit/gnu/machine?id=582cf9257cd1f9c969fbba5eb1c336ac8b975cde'>system: Add store-directory-prefix to boot-parameters.</a><span class='msg-avail'>...</span></td><td>Miguel Ángel Arruga Vivas</td></tr>
<tr><td><span title='2020-10-18 15:33:05 +0200'>2020-10-18</span></td><td><a href='/guix/commit/gnu/machine?id=eaf096398349a484bd23fd829755f7dfaf237ab4'>system: Provide locale information to the bootloader.</a><span class='msg-avail'>...</span></td><td>Miguel Ángel Arruga Vivas</td></tr>
<tr><td><span title='2020-09-07 11:00:48 +0200'>2020-09-07</span></td><td><a href='/guix/commit/gnu/machine?id=1c3b709edb8e6248a9a84dde57b27f3fdc997f21'>linux-boot: Handle nfs-root device strings.</a><span class='msg-avail'>...</span></td><td>Stefan</td></tr>
<tr><td><span title='2020-08-25 12:39:11 +0200'>2020-08-25</span></td><td><a href='/guix/commit/gnu/machine?id=dac7dd1b0b40c9f8c81b5147c68f6387c2b16bfd'>Remove "guile-zlib" extension when unused.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2020-08-25 11:53:20 +0200'>2020-08-25</span></td><td><a href='/guix/commit/gnu/machine?id=755f365b02b42a5d1e8ef3000dadef069553a478'>linux-libre: Support module compression.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2020-07-27 12:06:35 +0200'>2020-07-27</span></td><td><a href='/guix/commit/gnu/machine?id=a396dd01bc6e90ae512001350d1afa471e01661d'>machine: ssh: Check for potential system downgrades.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-07-25 19:11:37 +0200'>2020-07-25</span></td><td><a href='/guix/commit/gnu/machine?id=d51bfe242fbe6f3f8f71d723e8fe0c7bbe711ba1'>Use 'formatted-message' instead of '&amp;message' where appropriate.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-06-28 23:32:52 +0200'>2020-06-28</span></td><td><a href='/guix/commit/gnu/machine?id=fdbba544d8e05ac715a99267a81a127b91db4f21'>Add more missing (ice-9 format) imports.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2020-06-18 23:20:41 -0400'>2020-06-18</span></td><td><a href='/guix/commit/gnu/machine?id=0dd04b9986dd834782c988035569089785a6987d'>machine: ssh: Fix guix deploy hang when using non-DCE UUIDs.</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>