2.13.0 release

This release is fully compatible with the previous 2.12.1 version.

Enhancements:

• Add /data/demodularized/rpms list to modulemd and modulemd-packager formats.
  The listed binary RPM package names will be handled by a package manager as
  removed from all previous module versions. This enables moving a package from
  a module to a non-modular package set.
• modulemd-validator tool enables you to constrain a document type with a new
  "--type" option. Without this option, all document types recognized by
  a modulemd index are accepted. To validate version 3 of modulemd-packager
  format, use "modulemd-validator --type modulemd-packager-v3 foo.yaml"
  command.
• Allow disabling Python 3. You can pass -Dwith_py3=false to a meson setup
  command to disable building a binding for Python 3. The binding is enabled
  by default.

Fixes:

• Reject invalid integers. Purely non-numeric values and negative numbers where
  an unsigned type is mandated by a specification raise a parser error now.
• Handle a failed g_setenv() call in modulemd-validator.
• Prevent from dereferencing a NULL pointer when reporting invalid
  subdocuments.
• A build script now refuses to disable Glib introspection
  (-Dskip_introspection=true) while a Python binding is requested
  (-Dwith_py3=true).
• "modulemd-validator --version" command returns 0 exit code now.