Setup.py and style fixes

acme/acme/challenges.py

@@ -713,11 +713,12 @@ def txt_records_for_name(name):
     """
     try:
         import dns.resolver
+        import dns.exception
         dns_response = dns.resolver.query(name, 'TXT')
     except ImportError as error:
         raise ImportError("Local validation for 'dns-01' challenges requires "
-                          "'dnspython'");
-    except Exception as error:
+                          "'dnspython'")
+    except dns.exception.DNSException as error:
         logger.error("Unable to resolve %s: %s", name, str(error))
         return []
     return [txt_rec for rdata in dns_response for txt_rec in rdata.strings]

acme/setup.py

@@ -75,7 +75,7 @@ setup(
     install_requires=install_requires,
     extras_require={
         'docs': docs_extras,
-        'testing': testing_extras + 'dnspython',
+        'testing': testing_extras + ['dnspython'],
     },
     entry_points={
         'console_scripts': [