AddTextLetterByLetter animation no longer working

[cobordism]

Describe the bug
The AddTextLetterByLetter animation was broken by a recent PR
#568 (comment)

All the text appears immediately - even though self.mobject.submobjects is updated correctly in the ShowIncreasingSubsets Animation object.

To Reproduce

class lbl(Scene):
    def construct(self):
        t = Tex("this is a test")
        self.play(AddTextLetterByLetter(t[0]))
        #this works on the last releases branch (0.1.0), but not on master

Expected behavior
I expect the letters to appear on screen one at a time

[cobordism]

In the meantime, if y'all want to write text letter by letter, you can try something like this:

def LetterByLetter(txt):
    n = len(txt)
    return AnimationGroup(
        *[Write(txt[i], run_time=0.25) for i in range(n)], lag_ratio=0.9, run_time=0.25 * 0.9 * n
    )

perhaps replacing Write with something more abrupt. (Is there an add Animation just as there is (now) a Wait Animation?)