Upgrade werkzeug in test environment

[moy]

Since 41262fb we pin werkzeug to <2.1.0. Unfortunately, there's a vulnerability noted by snyk from [email protected] to [email protected], so this constraint forces us to be vulnerable.

It's not such a big deal since it's only for the test environment, but it would still be nice to upgrade.

Unfortunately, httpbin seems more or less unmaintained but fortunately, there's a fork and support for recent Werkzeug is on the TODO-list: psf/httpbin#35

Probably we should wait for the next release of the fork (hopefully also named httpbin on PyPI) and upgrade.