refactor: Clean "(P)" in front of Moody's ratings

[a-vester]

Some agencies attach a rating without having been requested by the issuer. This is usually indicated by the attachment of the letter u (unsolicited), e.g. AA+u.

Moody's uses (P) (public information only) before the actual rating, e.g. (P)P-1.

Add the possibility to use get_pure_ratings in order to clean (P).

pyratings/src/pyratings/clean.py

Lines 22 to 37 in 49a3025

	def get_pure_ratings(
	ratings: Union[str, pd.Series, pd.DataFrame]
	) -> Union[str, pd.Series, pd.DataFrame]:
	"""Remove rating watches/outlooks.
	Parameters
	----------
	ratings
	Rating may contain watch, such as `AA- *+`, `BBB+ (CwNegative)`.
	Outlook/watch should be seperated by a blank from the actual rating.
	Returns
	-------
	Union[str, pd.Series, pd.DataFrame]
	Regular ratings stripped off of watches. The name of the resulting Series or
	the columns of the returning DataFrame will be suffixed with `_clean`.