More permissive version constraint for pytz

As pytz is date-versioned, a caret requirement will mean any new release
will be incompatible with this, preventing users from upgrading. This
removes that constraint.

pyproject.toml

@@ -46,7 +46,7 @@ pytest-cov = {version="^3.0", optional=true}
 pytest-django = {version="^4.5.2", optional=true}
 pytest-pythonpath = {version="^0.7.3", optional=true}
 djet = {version="^0.3.0", optional=true}
-pytz = "^2021.3"
+pytz = ">=2021.3"
 webauthn = {version="<1.0", optional=true}