doc: handle release definition when not being in a git repository

doc/conf.py

@@ -66,7 +66,10 @@ copyright = u'2010-{}, Invisible Things Lab'.format(time.strftime('%Y'))
 # The short X.Y version.
 version = open('../version').read().strip()
 # The full version, including alpha/beta/rc tags.
-release = subprocess.check_output(['git', 'describe', '--long', '--dirty']).strip().decode()
+try:
+    release = subprocess.check_output(['git', 'describe', '--long', '--dirty']).strip().decode()
+except:
+    release = "1"
 
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