pynacl is missing the cffi backend

Python 3.6.5, 64 Bit
Pyinstaller develop.zip version

the project uses pynacl, however the cffi backend does not get included.

Traceback (most recent call last):
  File "c:\program files\python36\lib\site-packages\PyInstaller\loader\pyimod03_importers.py", line 712, in load_module
    module = loader.load_module(fullname)
  File "<frozen importlib._bootstrap_external>", line 399, in _check_name_wrapper
  File "<frozen importlib._bootstrap_external>", line 823, in load_module
  File "<frozen importlib._bootstrap_external>", line 682, in load_module
  File "<frozen importlib._bootstrap>", line 265, in _load_module_shim
  File "<frozen importlib._bootstrap>", line 684, in _load
  File "<frozen importlib._bootstrap>", line 658, in _load_unlocked
  File "<frozen importlib._bootstrap>", line 571, in module_from_spec
  File "<frozen importlib._bootstrap_external>", line 922, in create_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
ModuleNotFoundError: No module named '_cffi_backend'

During handling of the above exception, another exception occurred:


Traceback (most recent call last):
  File "main.py", line 98, in <module>
  File "main.py", line 36, in check_licence
  File "c:\program files\python36\lib\site-packages\PyInstaller\loader\pyimod03_importers.py", line 628, in exec_module
    exec(bytecode, module.__dict__)
  File "site-packages\nacl\signing.py", line 19, in <module>
  File "c:\program files\python36\lib\site-packages\PyInstaller\loader\pyimod03_importers.py", line 628, in exec_module
    exec(bytecode, module.__dict__)
  File "site-packages\nacl\bindings\__init__.py", line 17, in <module>
  File "c:\program files\python36\lib\site-packages\PyInstaller\loader\pyimod03_importers.py", line 628, in exec_module
    exec(bytecode, module.__dict__)
  File "site-packages\nacl\bindings\crypto_box.py", line 18, in <module>
  File "c:\program files\python36\lib\site-packages\PyInstaller\loader\pyimod03_importers.py", line 714, in load_module
    raise ImportError('%s: %s' % (e, fullname))
ImportError: No module named '_cffi_backend': nacl._sodium
[2768] Failed to execute script main

Reproduce the failure with the reported Python 3.6.5 setup and PyInstaller develop.zip, starting from the loader traceback and site-packages/nacl/bindings/crypto_box.py. Trace how the PyInstaller bundle discovers nacl._sodium and _cffi_backend; done means the bundled application imports PyNaCl without the reported ModuleNotFoundError.