Only install monotonic on python2 (#583)

Only install monotonic on python<3.5

Co-authored-by: Sergey Shepelev <temotor@gmail.com>

eventlet/__init__.py

@@ -21,8 +21,11 @@
     # Helpful when CPython < 3.5 on Linux blocked in `os.waitpid(-1)` before first use of hub.
     # Example: gunicorn
     # https://github.com/eventlet/eventlet/issues/401#issuecomment-327500352
-    import monotonic
-    del monotonic
+    try:
+        import monotonic
+        del monotonic
+    except ImportError:
+        pass
 
     connect = convenience.connect
     listen = convenience.listen

eventlet/hubs/hub.py

@@ -22,7 +22,11 @@ def alarm_signal(seconds):
 import eventlet.hubs
 from eventlet.hubs import timer
 from eventlet.support import greenlets as greenlet, clear_sys_exc_info
-import monotonic
+try:
+    from monotonic import monotonic
+except ImportError:
+    from time import monotonic
+
 import six
 
 g_prevent_multiple_readers = True
@@ -120,7 +124,7 @@ def __init__(self, clock=None):
         self.closed = []
 
         if clock is None:
-            clock = monotonic.monotonic
+            clock = monotonic
         self.clock = clock
 
         self.greenlet = greenlet.greenlet(self.run)

setup.py

@@ -17,7 +17,7 @@
     install_requires=(
         'dnspython >= 1.15.0, < 2.0.0',
         'greenlet >= 0.3',
-        'monotonic >= 1.4',
+        'monotonic >= 1.4;python_version<"3.5"',
         'six >= 1.10.0',
     ),
     zip_safe=False,