setup: bump webtest minimum version to 2.0.6 to get py3 stdlib support

/usr/lib64/python3.7/http/cookiejar.py:723: in is_third_party
    if not domain_match(req_host, reach(request.origin_req_host)):
E   AttributeError: '_RequestCookieAdapter' object has no attribute 'origin_req_host'

dev_requirements.txt

@@ -4,5 +4,5 b' pytest-benchmark >= 3.2.2, < 3.3'
 pytest-localserver >= 0.5.0, < 0.6
 mock >= 3.0.0, < 3.1
 Sphinx >= 1.8.0, < 1.9
-WebTest >= 2.0.3, < 2.1
+WebTest >= 2.0.6, < 2.1
 isort == 4.3.21