Backport "Some fixes for Fedora 34" and more into 1.18

This backports parts of !716 (merged) and !622 (merged), contains the following commits:

98c9296b gobject-introspection: Backport patch for Python 3.9 compat, fixes #359 (closed)

a6d6711e srt.recipe: Bump to 1.4.3, fixes build on Fedora 34

bd71f936 glib-networking: Update options and backport c_std=c99

b0c71fea cerbero: Don't try to git commit if nothing to commit

a05e5373 cerbero: Do not warn about non-existent cache file