Update users.rst

Add hollerith

Author: koubaa

docs/users.rst

@@ -36,6 +36,11 @@ Examples of projects using pythoncapi_compat.h
   Mercurial extension.
 * `python-zstd <https://github.com/sergey-dryabzhinsky/python-zstd/>`_
   (`commit <https://github.com/sergey-dryabzhinsky/python-zstd/commit/8aa6d7a4b250e1f0a4e27b4107c39dc516c87f96>`__)
+* `hollerith <https://github.com/pyansys/hollerith/>`_
+  ``src/writer.c`` uses ``PyObject_CallOneArg() and other Python 3.9 apis``
+   (`pythoncapi_compat.h copy
+  <https://github.com/pyansys/hollerith/blob/main/src/pythoncapi_compat.h>`__)
+
 
 
 Projects not using pythoncapi_compat.h