Remove incorrect upper bound from pytz dependency

Change the `pytz` dependency to use the `>=` operator rather than `^`,
in order to allow installing newer versions.  `pytz` uses calendar
versioning rather than semantic versioning, so the `^` operator is
meaningless there.  Furthermore, since `pytz` version corresponds
to the timezone data release, allowing the newest version is important
to permit using up-to-date tzinfo.

pyproject.toml

@@ -30,7 +30,7 @@ include = [
 
 [tool.poetry.dependencies]
 python = "^3.7"
-pytz = "^2022.7"
+pytz = ">=2022.7"
 Flask = "^2.0.0"
 Babel = "^2.11.0"
 Jinja2 = "^3.1.2"