bpo-46784: Add newly exported expat symbols to the namespace. (GH-31397)

The libexpat 2.4.1 upgrade from  introduced the following new exported symbols:

* `testingAccountingGetCountBytesDirect`
* `testingAccountingGetCountBytesIndirect`
* `unsignedCharToPrintable`
* `XML_SetBillionLaughsAttackProtectionActivationThreshold`
* `XML_SetBillionLaughsAttackProtectionMaximumAmplification`

We need to adjust [Modules/expat/pyexpatns.h](https://github.com/python/cpython/blob/master/Modules/expat/pyexpatns.h)

(The newer libexpat upgrade  has no new symbols).

Automerge-Triggered-By: GH:gpshead