Make from_str a staticmethod

cls is never used.

Author: sebastinas

curtsies/formatstring.py

@@ -290,8 +290,8 @@ def __init__(self, *components: Chunk) -> None:
         self._s: Optional[str] = None
         self._width: Optional[int] = None
 
-    @classmethod
-    def from_str(cls, s: str) -> "FmtStr":
+    @staticmethod
+    def from_str(s: str) -> "FmtStr":
         r"""
         Return a FmtStr representing input.