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

[junkmd]

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

comtypes/comtypes/automation.py

Lines 551 to 556 in 6c1788a

	v.vt = VT_ERROR
	if sys.version_info >= (3, 0):
	v._.VT_I4 = 0x80020004
	else:
	v._.VT_I4 = 0x80020004L
	del v

comtypes/comtypes/automation.py

Lines 551 to 553 in 2e94dbc

	v.vt = VT_ERROR
	v._.VT_I4 = 0x80020004
	del v

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.