Can't lock some pipfiles

[greenape]

I think as a result of:

• https://github.com/pypa/pipenv/issues/4553

This is 'hidden' to us in some cases, but manifested for me when changing the pins on flowclient dependencies and then relocating integration tests - with pipenv > 2018, I can't relock, because the version of flowclient actually being used is not the local one, but the one on pypi.

The 'fix' would appear to be to use pipenv==2018.11.26, but I think that is not satisfactory - not least because it will break if we upgrade to python 3.10.