The pkg-config file for Python3 gets a different name depending on the way it is built: - Configuring with --extra-python=python3 installs a file pytalloc-util-cp36m.pc - Configuring with Python3 as the only / primary Python, you get a file pytalloc-util.pc. I assume that this is not intended, given the effort done for the very specific symbols files.
Fixed, if only because --extra-python= is gone.