fixed typo in setup call: extra_require->extras_require

Author: oleksandr-pavlyk

setup.py

@@ -97,7 +97,7 @@ def _patched_copy_file(src_file, dest_file, hide_listing=True):
     install_requires=[
         "numpy",
     ],
-    extra_require={
+    extras_require={
         "docs": [
             "Cython",
             "sphinx",