pyrsistent install fails on py2.7

The python2.7 installation of pyresistent (a jsonschema dependency)
fails with the following error message:

"pyrsistent requires Python '>=3.5' but the running Python is 2.7.15"

Pyrsistent reportedly dropped python2.7 support [1]. Py2.7 installs
of jsonschema must pin the latest pyresistent version with python2.7
support (pyrsistent-0.16.1) to avoid an installation error.

[1] tobgu/pyrsistent#208

test-requirements.txt

@@ -1,4 +1,5 @@
 pytest
+pyrsistent==0.16.1
 jsonschema
 PyYAML
 mock