Pinning pip because new version does not work with some versions of Celery and Httpx (#3195)

Installing Celery 5.1.x (and older HTTPX versions) requirements in our tests does not work with the new pip 24.1.
Downgrading pip to 24.0 for now.

Author: antonpirker

tox.ini

@@ -4,6 +4,9 @@
 # and then run "tox" from this directory.
 
 [tox]
+requires =
+    # This version introduced using pip 24.1 which does not work with older Celery and HTTPX versions.
+    virtualenv<20.26.3  
 envlist =
     # === Common ===
     {py3.6,py3.7,py3.8,py3.9,py3.10,py3.11,py3.12}-common