Making PyPi markdown friendly requires additional property (#11093)

* updating python markdown property

* adding to samples

samples/openapi3/client/petstore/python-legacy/setup.py

@@ -35,6 +35,7 @@ setup(
     packages=find_packages(exclude=["test", "tests"]),
     include_package_data=True,
     license="Apache-2.0",
+    long_description_content_type='text/markdown',
     long_description="""\
     This spec is mainly for testing Petstore server and contains fake endpoints, models. Please do not use this for any other purpose. Special characters: \" \\  # noqa: E501
     """