Fix deprecation warning for resolver.query()

/home/travis/build/linkchecker/linkchecker/linkcheck/checker/mailtourl.py:321: DeprecationWarning: please use dns.resolver.resolve() instead
    answers = resolver.query(domain, 'MX')

linkcheck/checker/mailtourl.py

@@ -319,7 +319,7 @@ class MailtoUrl(urlbase.UrlBase):
         username, domain = mail.rsplit('@', 1)
         log.debug(LOG_CHECK, "looking up MX mailhost %r", domain)
         try:
-            answers = resolver.query(domain, 'MX')
+            answers = resolver.resolve(domain, 'MX', search=True)
         except DNSException:
             answers = []
         if len(answers) == 0:

requirements.txt

@@ -2,6 +2,6 @@
 beautifulsoup4
 requests >= 2.4
 pyxdg
-dnspython
+dnspython >= 2.0
 # optional:
 argcomplete

setup.py

@@ -378,7 +378,7 @@ setup(
     python_requires=">= 3.6",
     install_requires=[
         "requests >= 2.4",
-        "dnspython",
+        "dnspython >= 2.0",
         "beautifulsoup4",
         "pyxdg",
     ],

tox.ini

@@ -30,7 +30,7 @@ deps =
     beautifulsoup4 < 4.8.1
     requests >= 2.4
     pyxdg
-    dnspython
+    dnspython >= 2.0
     {[base]deps}
 
 [testenv:flake8]