Fix usage of deprecated startdir parameter

Use the start_path parameter instead of startdir, since Pytest 7 yield
the following deprecation warning if startdir is being used:

    site-packages/pytest_sugar.py:279: PytestRemovedIn8Warning:
    The (startdir: py.path.local) argument is deprecated, please
    use (start_path: pathlib.Path)

Details of the Pytest deprecation can be found from:
https://docs.pytest.org/en/latest/deprecations.html#py-path-local-arguments-for-hooks-replaced-with-pathlib-path

Fixes #232

pytest_sugar.py

@@ -277,7 +277,7 @@ def pytest_sessionstart(self, session: Session) -> None:
             bold=True,
         )
         lines = self.config.hook.pytest_report_header(
-            config=self.config, startdir=self.startpath
+            config=self.config, start_path=self.startpath
         )
         lines.reverse()
         for line in flatten(lines):