_convert: add type hints

Because of the underscore, _convert is (correctly) not part of the
external API, so this is for internal typechecking purposes only.  The
.pyi file will still be installed though.

Signed-off-by: Benjamin Gilbert <bgilbert@cs.cmu.edu>

Author: bgilbert

openslide/_convert.pyi

@@ -0,0 +1,26 @@
+#
+# openslide-python - Python bindings for the OpenSlide library
+#
+# Copyright (c) 2024 Benjamin Gilbert
+#
+# This library is free software; you can redistribute it and/or modify it
+# under the terms of version 2.1 of the GNU Lesser General Public License
+# as published by the Free Software Foundation.
+#
+# This library is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+# or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
+# License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with this library; if not, write to the Free Software Foundation,
+# Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+#
+
+from typing import Protocol
+
+class _Buffer(Protocol):
+    # Python 3.12+ has collections.abc.Buffer
+    def __buffer__(self, flags: int) -> memoryview: ...
+
+def argb2rgba(buf: _Buffer) -> None: ...