Various CI jobs fail because of pyrsistent 0.17.2

[exarkun]

pyrsistent 0.17.0 dropped Python 2 support (https://github.com/tobgu/pyrsistent/blob/fed6c1781a915b60b731051d294fe4b50d26de7a/CHANGES.txt#L20) but failed to declare this in its package metadata, breaking installation on Python 2.

pyrsistent 0.17.1 and 0.17.2 tried to fix this but, apparently, failed. The notes for 0.17.3 claim it really fixes the issue. Then, 0.17.0 through 0.17.2 were yanked from pypi.

However, many of our CI jobs fail during environment setup with an error like:

Collecting pyrsistent>=0.11.8 (from eliot~=1.7->magic-folder==0.0.0.post1521)
  Downloading https://files.pythonhosted.org/packages/7d/ae/90ddcf28fb8eee5d4990920586d2856342e42faa95f39223f0b9762ef264/pyrsistent-0.17.2.tar.gz (106kB)
ERROR: pyrsistent requires Python '>=3.5' but the running Python is 2.7.13
WARNING: You are using pip version 19.1.1, however version 20.2.3 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.

So 0.17.2 is still causing problems for us somehow.

[exarkun]

python-jsonschema/jsonschema#747 (comment) seems to supply the explanation