remove dependency on IPython genutils (#238)

docs/requirements.txt

@@ -1,4 +1,3 @@
-ipython_genutils
 jsonschema
 traitlets
 -e git+https://github.com/jupyter/jupyter_core.git#egg=jupyter_core

nbformat/_imports.py

@@ -0,0 +1,40 @@
+"""
+A simple utility to import something by its string name.
+
+Vendored form ipython_genutils
+"""
+
+# Copyright (c) IPython Development Team.
+# Distributed under the terms of the Modified BSD License.
+
+
+def import_item(name):
+    """Import and return ``bar`` given the string ``foo.bar``.
+
+    Calling ``bar = import_item("foo.bar")`` is the functional equivalent of
+    executing the code ``from foo import bar``.
+
+    Parameters
+    ----------
+    name : string
+      The fully qualified name of the module/package being imported.
+
+    Returns
+    -------
+    mod : module object
+       The module that was imported.
+    """
+
+    parts = name.rsplit(".", 1)
+    if len(parts) == 2:
+        # called with 'foo.bar....'
+        package, obj = parts
+        module = __import__(package, fromlist=[obj])
+        try:
+            pak = getattr(module, obj)
+        except AttributeError:
+            raise ImportError("No module named %s" % obj)
+        return pak
+    else:
+        # called with un-dotted string
+        return __import__(parts[0])