Rename io.py to gt_io.py

This avoids confusion with the standard 'io' module.

src/graph_tool/Makefile.am

@@ -4,7 +4,7 @@ graph_tool_PYTHON = \
     __init__.py \
     dl_import.py \
     decorators.py \
-    io.py \
+    gt_io.py \
     all.py
 
 graph_tooldir = $(MOD_DIR)

src/graph_tool/__init__.py

@@ -105,7 +105,7 @@ from .dl_import import *
 dl_import("from . import libgraph_tool_core as libcore")
 __version__ = libcore.mod_info().version
 
-from . import io  # sets up libcore io routines
+from . import gt_io  # sets up libcore io routines
 
 import sys
 import os