doc: do not depend on typed-ast

doc: do not depend on typed-ast

the typed-ast project was marked end of life since July 2023, and
not maintained anymore. since we build the document using readthedocs'
service, and in .readtherdocs.yml we use python 3.9, which comes with
ast module included by its standard library.

the typed-ast dependency was originally added in 30d41597, but now that
we are using python 3.9, there is no need to use this module anymore.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>