build: Remove pytest pin in requirements-devenv.txt (getsentry#3696)

The pytest pin in requirements-devenv.txt appears to be unnecessary. Our tests anyways do not seem to respect this pin; the actual pins are defined for each environment in tox.ini.

ref getsentry#3035

requirements-devenv.txt

@@ -1,5 +1,5 @@
 -r requirements-linting.txt
 -r requirements-testing.txt
 mockupdb # required by `pymongo` tests that are enabled by `pymongo` from linter requirements
-pytest<7.0.0 # https://github.com/pytest-dev/pytest/issues/9621; see tox.ini
+pytest
 pytest-asyncio