[xml] Release of libxml2 2.10.1

[Nick Wellnhofer <wellnhofer aevum de>]

Version 2.10.1 of libxml2 is available at:

    https://download.gnome.org/sources/libxml2/2.10/

This fixes some showstoppers reported by early adopters of 2.10.0. Thsnk for 
the reports!

### Regressions

- Fix xmlCtxtReadDoc with encoding

### Bug fixes

- Fix HTML parser with threads and --without-legacy

### Build system

- Fix build with Python 3.10
- cmake: Disable version script on macOS
- Remove Makefile rule to build testapi.c

### Documentation

- Switch back to HTML output for API documentation
- Port doc/examples/index.py to Python 3
- Fix order of exports in libxml2-api.xml
- Remove libxml2-refs.xml

Nick