Only build monotonic on py2

setup/build.py

@@ -27,6 +27,7 @@ class Extension(object):
         self.name = d['name'] = name
         self.sources = d['sources'] = absolutize(sources)
         self.needs_cxx = d['needs_cxx'] = bool([1 for x in self.sources if os.path.splitext(x)[1] in ('.cpp', '.c++', '.cxx')])
+        self.needs_py2 = d['needs_py2'] = kwargs.get('needs_py2', False)
         self.headers = d['headers'] = absolutize(kwargs.get('headers', []))
         self.sip_files = d['sip_files'] = absolutize(kwargs.get('sip_files', []))
         self.inc_dirs = d['inc_dirs'] = absolutize(kwargs.get('inc_dirs', []))
@@ -262,6 +263,8 @@ class Build(Command):
         for ext in extensions:
             if opts.only != 'all' and opts.only != ext.name:
                 continue
+            if ext.needs_py2 and sys.version_info >= (3,):
+                continue
             if ext.error:
                 if ext.optional:
                     self.warn(ext.error)

setup/extensions.json

@@ -10,7 +10,8 @@
     {
         "name": "monotonic",
         "sources": "calibre/utils/monotonic.c",
-        "linux_libraries": "rt"
+        "linux_libraries": "rt",
+        "needs_py2": true
     },
     {
         "name": "unicode_names",