TYP: chunksize, but ignored assignment check

For some reason mypy would not recognize that chunksize
turns from Optional[int] to int inside the setter.
Even setting an intentional assertion
``assert chunksize is not None``
does not help.

Author: ivanovmg

pandas/io/formats/csvs.py

@@ -112,7 +112,7 @@ def __init__(
         self.line_terminator = line_terminator or os.linesep
         self.date_format = date_format
         self.cols = cols
-        self.chunksize = chunksize
+        self.chunksize = chunksize  # type: ignore[assignment]
 
     @property
     def index_label(self) -> IndexLabel:
@@ -197,13 +197,14 @@ def _number_format(self) -> dict:
         )
 
     @property
-    def chunksize(self):
+    def chunksize(self) -> int:
         return self._chunksize
 
     @chunksize.setter
-    def chunksize(self, chunksize):
+    def chunksize(self, chunksize: Optional[int]) -> None:
         if chunksize is None:
             chunksize = (100000 // (len(self.cols) or 1)) or 1
+        assert chunksize is not None
         self._chunksize = int(chunksize)
 
     @property