Compare the types of the filename parameters between

and

If we trace the use of the filename parameter through BaseDocTemplate, we go through:

• In build(): used in the call to _startBuild()
• In _startBuild(): is used in the call to _makeCanvas
• In _makeCanvas(): Is used as one alternative in a parameter passed into (by default) canvas.Canvas, and the other alternative is self.filename, which is populated in BaseDocTemplate.__init__().
• It is not used elsewhere in the call tree.

Therefore, the type of the filename parameter of BaseDocTemplate.build() should be str | IO[Bytes] | None. It also matches the type of the parameter of Canvas.__init__().

I've put a minimal test for this issue in a gist: https://gist.github.com/egerlach/f47c9722794b5d03f832aada27da2293