Since switching to the buildthings backend, running tox in djblets was

crashing with an assertion "Attempted to patch dependencies too late!"
The cause here is that pyproject_api invokes build hooks multiple times

in a single worker process, so the earlier get_requires_for_build_*

hooks instantiate a _ConfigExpander (via Distribution) before

prepare_metadata_for_build_wheel calls

patch_setuptools_package_deps(). The "already initialized" assertion

fired in that case, even though the next _ConfigExpander instance (the

one we actually want to patch) has not been built yet.
This change gets rid of the assertion, and clears the post-funcs list

before appending so deps don't stack across hook calls in the same

worker.

Built a package and put it in my $PIP_FIND_LINKS. Was able to run

tox successfully.
Ran unit tests.