Check version requirements on optional dependencies (#3618)

* Add and test activate function to acme.

This function can be used to check if our optional dependencies are
available and they meet our version requirements.

* use activate in dns_resolver

* use activate in dns_available() in challenges_test

* Use activate in dns_resolver_test

* Use activate in certbot.plugins.util_test

* Use acme.util.activate for psutil

* Better testing and handling of missing deps

* Factored out *_available() code into a common function

* Delayed exception caused from using acme.dns_resolver without
  dnspython until the function is called. This makes both
  production and testing code simpler.

* Make a common subclass for already_listening tests

* Simplify mocking of USE_PSUTIL in tests

certbot/plugins/util.py

@@ -5,13 +5,19 @@ import socket
 
 import zope.component
 
+from acme import errors as acme_errors
+from acme import util as acme_util
+
 from certbot import interfaces
 from certbot import util
 
+PSUTIL_REQUIREMENT = "psutil>=2.2.1"
+
 try:
-    import psutil
+    acme_util.activate(PSUTIL_REQUIREMENT)
+    import psutil  # pragma: no cover
     USE_PSUTIL = True
-except ImportError:
+except acme_errors.DependencyError:  # pragma: no cover
     USE_PSUTIL = False
 
 logger = logging.getLogger(__name__)