ritest: WLS support for `ritest()`

[s3alfisc]

Context

Currently, the ritest method does not support WLS.

To do

• Enable WLS support for ritest.
• Read self._weights attribute from regression object.
• adjust _get_ristest_stats_slow (easy) and _get_ritest_stats_fast (slightly more complex) to correctly handle WLS.
• implement tests against R's ritest by adding tests with stratification by adjusting https://github.com/py-econometrics/pyfixest/blob/734a21c34a56a88ba12b0fa26cd3ed0e8c24bd51/tests/r_test_comparisons.R#L13
• add tests to tests/test_ritest.py