CDRIVER-4767 Support Python 3.6 in man page build (#1504)

* remove use of `:=`

Assignment expressions were introduced in Python 3.8

* use `typing.Union` instead of `|`

Writing union types as `X | Y` was introduced in Python 3.10.

* do not use Union `|` operator

The Union `|` operator was added in Python 3.9. See: PEP 584

* use `typing.List` and `typing.Dict`

Type hinting for `list`, `dict` was added in Python 3.9. See: PEP 585