Fix dlopen() flags problem with Python 3.6

src/graph_tool/dl_import.py

@@ -24,17 +24,21 @@ import sys
 import os.path
 
 try:
-    from DLFCN import RTLD_LAZY, RTLD_GLOBAL
+    from os import RTLD_LAZY, RTLD_GLOBAL
     dl_flags = RTLD_LAZY | RTLD_GLOBAL
 except ImportError:
-    # handle strange python installations, by importing from the deprecated dl
-    # module, otherwise from ctypes
     try:
-        from dl import RTLD_LAZY, RTLD_GLOBAL
+        from DLFCN import RTLD_LAZY, RTLD_GLOBAL
         dl_flags = RTLD_LAZY | RTLD_GLOBAL
     except ImportError:
-        from ctypes import RTLD_GLOBAL
-        dl_flags = RTLD_GLOBAL
+        # handle strange python installations, by importing from the deprecated dl
+        # module, otherwise from ctypes
+        try:
+            from dl import RTLD_LAZY, RTLD_GLOBAL
+            dl_flags = RTLD_LAZY | RTLD_GLOBAL
+        except ImportError:
+            from ctypes import RTLD_GLOBAL
+            dl_flags = RTLD_GLOBAL
 
 __all__ = ["dl_import"]