Allow get_include to handle sdist installations

Author: JDBetteridge

Diff for: rtree/core.py

@@ -75,8 +75,9 @@ def free_error_msg_ptr(result, func, cargs):
 
 def get_include():
     import rtree
-
-    return os.path.join(os.path.dirname(rtree.__file__), "include")
+    from importlib import resources
+    include_dir = resources.files(rtree).joinpath('include')
+    return str(include_dir) if include_dir.is_dir() else ''
 
 
 # load the shared library by looking in likely places