MAINT: Make app setup optional in __init__.

Author: rossbar

numpydoc/__init__.py

@@ -6,7 +6,16 @@
 from ._version import __version__
 
 
-def setup(app, *args, **kwargs):
-    from .numpydoc import setup
+# NOTE: Determine whether sphinx is installed with an explicit import.
+# If so, define the setup function for registering the numpydoc extension;
+# otherwise skip this step.
+try:
+    import sphinx
 
-    return setup(app, *args, **kwargs)
+
+    def setup(app, *args, **kwargs):
+        from .numpydoc import setup
+
+        return setup(app, *args, **kwargs)
+except ModuleNotFoundError:
+    pass