other approach fails because we cannot upgrade pytest to fix attrs
incompatiblity because pytest has dropped python 2.7 support in 5.x series

Newer typing in nixpkgs requires newer Hypothesis than it has.

openssl_1_1 in the pinned version of nixpkgs is 1.1.1c.

* switch from tahoe-lafs src on my laptop to github
* package required updated versions of tahoe-lafs dependencies
* use overlays to avoid version skew between different python libs
* copy/paste/edit the nixpkgs expressions instead of overrideAttrs because the
  latter doesn't converge, I guess?

Not currently working