Move dnspython dependency to tests only and only import the dns.resolver when

actually resolving the client.  That way user code that does not call
'simple_verify' for DNS01 challenges does not depend on dnspython.

acme/acme/challenges.py

@@ -6,8 +6,6 @@ import logging
 import socket
 
 from cryptography.hazmat.primitives import hashes
-import dns.resolver
-import dns.exception
 import OpenSSL
 import requests
 
@@ -714,8 +712,12 @@ def txt_records_for_name(name):
 
     """
     try:
+        import dns.resolver
         dns_response = dns.resolver.query(name, 'TXT')
-    except dns.exception.DNSException as error:
-        logger.error("Unable to resolve %s: %s", name, error)
+    except ImportError as error:
+        raise ImportError("Local validation for 'dns-01' challenges requires "
+                          "'dnspython'");
+    except Exception 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

@@ -12,7 +12,6 @@ install_requires = [
     'cryptography>=0.8',
     # Connection.set_tlsext_host_name (>=0.13), X509Req.get_extensions (>=0.15)
     'PyOpenSSL>=0.15',
-    'dnspython',
     'pyrfc3339',
     'pytz',
     'requests',
@@ -76,7 +75,7 @@ setup(
     install_requires=install_requires,
     extras_require={
         'docs': docs_extras,
-        'testing': testing_extras,
+        'testing': testing_extras + 'dnspython',
     },
     entry_points={
         'console_scripts': [