Fix deserialization with msgpack 1.0.

Patch copied from upstream source repository:
https://github.com/kovidgoyal/calibre/commit/0ff41ac64994ec11b7859fc004c94d08769e3af3

diff --git a/src/calibre/utils/serialize.py b/src/calibre/utils/serialize.py
index f5d560c468..c35ae53849 100644
--- a/src/calibre/utils/serialize.py
+++ b/src/calibre/utils/serialize.py
@@ -110,7 +110,7 @@ def msgpack_decoder(code, data):
 def msgpack_loads(dump, use_list=True):
     # use_list controls whether msgpack arrays are unpacked as lists or tuples
     import msgpack
-    return msgpack.unpackb(dump, ext_hook=msgpack_decoder, raw=False, use_list=use_list)
+    return msgpack.unpackb(dump, ext_hook=msgpack_decoder, raw=False, use_list=use_list, strict_map_key=False)
 
 
 def json_loads(data):
n>
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<div class='path'>path: <a href='/guix/log/?id=773ef90517dba73b350479d1004b96d436af1617'>root</a>/<a href='/guix/log/tests?id=773ef90517dba73b350479d1004b96d436af1617'>tests</a>/<a href='/guix/log/tests/store-roots.scm?id=773ef90517dba73b350479d1004b96d436af1617'>store-roots.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/store-roots.scm?id=773ef90517dba73b350479d1004b96d436af1617&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2019-04-10 17:09:47 +0200'>2019-04-10</span></td><td><a href='/guix/commit/tests/store-roots.scm?id=72eda0624be89ed18302fd7d7f22976071ab020c'>Add (guix store roots).</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/store/roots.scm, tests/store-roots.scm: New files.
* Makefile.am (STORE_MODULES): Add guix/store/roots.scm.
(SCM_TESTS): Add tests/store-roots.scm.
</span></span></td><td>Ludovic Courtès</td></tr>