Replace the integer literal assigned to `VARIANT.missing` with a constant.

Assigning `0x80020004` to `VARIANT.missing` may not be appropriate.

https://github.com/enthought/comtypes/blob/6c1788afb7789b91959fb8ecb039538c90ca0754/comtypes/automation.py#L551-L556

https://github.com/enthought/comtypes/blob/2e94dbca2a579ac0a4b2f9388e5cdca7b113f480/comtypes/automation.py#L551-L553

The line is intended to assign `DISP_E_PARAMNOTFOUND` to a `VARIANT` field to create an instance for missing parameters.
`0x80020004` is evaluated as an unsigned 32-bit hexadecimal integer in Python, so it is `2147549188`, thus it does not match `DISP_E_PARAMNOTFOUND`(`-2147352572`).

Since neither `-2147352572` nor `2147549188` has caused any errors or behavioral differences in `VARIANT`, assigning `VT_ERROR` to the `vt` field might be the more crucial aspect.
Regardless, this part is highly non-obvious, and this is confusing.