Can't install flit_core in an isolated environment using a private repository

[nadavaviv]

I'm trying to install flit_core in an AWS instance which is not accessible to the pypi.org. I had to upload it to a private repository using s3pypi.

When trying to install flit I get the following error:

Traceback (most recent call last):
  File "/opt/conda/lib/python3.7/site-packages/pip/_internal/cli/base_command.py", line 188, in main
    status = self.run(options, args)
  File "/opt/conda/lib/python3.7/site-packages/pip/_internal/commands/install.py", line 345, in run
    resolver.resolve(requirement_set)
  File "/opt/conda/lib/python3.7/site-packages/pip/_internal/legacy_resolve.py", line 196, in resolve
    self._resolve_one(requirement_set, req)
  File "/opt/conda/lib/python3.7/site-packages/pip/_internal/legacy_resolve.py", line 359, in _resolve_one
    abstract_dist = self._get_abstract_dist_for(req_to_install)
  File "/opt/conda/lib/python3.7/site-packages/pip/_internal/legacy_resolve.py", line 307, in _get_abstract_dist_for
    self.require_hashes
  File "/opt/conda/lib/python3.7/site-packages/pip/_internal/operations/prepare.py", line 215, in prepare_linked_requirement
    finder, self.build_isolation,
  File "/opt/conda/lib/python3.7/site-packages/pip/_internal/distributions/source.py", line 70, in prepare_distribution_metadata
    reqs = self.req.pep517_backend.get_requires_for_build_wheel()
  File "/opt/conda/lib/python3.7/site-packages/pip/_vendor/pep517/wrappers.py", line 71, in get_requires_for_build_wheel
    'config_settings': config_settings
  File "/opt/conda/lib/python3.7/site-packages/pip/_vendor/pep517/wrappers.py", line 162, in _call_hook
    raise BackendUnavailable
pip._vendor.pep517.wrappers.BackendUnavailable

Do you maybe have a way I could work around this?

Thanks

[takluyver]

Similar to https://github.com/pypa/pip/issues/6100, it looks like pip & pep517 are hiding the real error details of what went wrong here.