Remove pyrsistent from test dependencies (#4588)

The `pyrsistent` dependency in `requirements-testing.txt` does not
appear to be used anywhere, so I removed it.

In Fedora, I maintain the [`python-pyrsistent`
package](https://src.fedoraproject.org/rpms/python-pyrsistent), and
[`python-sentry-sdk`](https://src.fedoraproject.org/rpms/python-sentry-sdk)
is the only thing that still depends on it. Since this dependency
appears to be vestigial, my goal is to remove it both upstream and
downstream and then orphan `python-pyrsistent` in Fedora as an unused
leaf package.

Author: musicinmybrain

requirements-testing.txt

@@ -6,7 +6,6 @@ pytest-forked
 pytest-localserver
 pytest-watch
 jsonschema
-pyrsistent
 executing
 asttokens
 responses