Fix for #1677 (#1678)

* Make `as_markup` return `super().as_markup`

* Add 1677.misc.rst

* Update 1677.misc.rst

CHANGES/1677.misc.rst

@@ -0,0 +1,5 @@
+Fixed MyPy [return-value] error in `InlineKeyboardBuilder().as_markup()`.
+`as_markup` method now overloads parent class method and uses `super()`, to call parent's 
+`as_markup` method.
+Also added correct type hint to `as_markup`'s return in `InlineKeyboardBuilder` and 
+`ReplyKeyboardBuilder` classes.