Drop Python 2 support, fix forward-compatibility

• CI: Disable documentation build with Python 3.10

  The version of sphinx in Ubuntu impish is not currently fully compatible with Python 3.10.

• build: Use sysconfig in preference to distutils.util

  distutils is deprecated and likely to be removed in Python 3.12.

• Remove support for Python 2

  Python 2 reached EOL on 2020-01-01, and the latest version of AX_PYTHON_DEVEL breaks the build with Python 2. This seems as good a time as any to drop compatibility.

• m4: Update AX_PYTHON_DEVEL for better forward-compatibility

• build: Make sure distcheck installs into our ${prefix}

• build: Use LT_INIT instead of deprecated AC_PROG_LIBTOOL

  This has been available since at least libtool 1.9b (2004).

• build: Update bug reporting URL

• .gitignore: Ignore build-system backup files

• build: Use AS_HELP_STRING instead of deprecated AC_HELP_STRING

  This has been available since at least autoconf 2.59c (2006).