Added requirements for coverage and lint.

acme/acme/challenges.py

@@ -245,7 +245,7 @@ class DNS01Response(KeyAuthorizationChallengeResponse):
             from acme import dns_resolver
             txt_records = dns_resolver.txt_records_for_name(
                 validation_domain_name)
-        except ImportError as error:
+        except ImportError:
             raise ImportError("Local validation for 'dns-01' challenges "
                               "requires 'dnspython'")
         exists = validation in txt_records

acme/setup.py

@@ -74,7 +74,9 @@ setup(
     include_package_data=True,
     install_requires=install_requires,
     extras_require={
+        'coverage': ['dnspython'],
         'docs': docs_extras,
+        'lint': ['dnspython'],
         'testing': testing_extras + ['dnspython'],
     },
     entry_points={