Add flake8 to travis and fix its errors

Author: alejandrogallo

.travis.yml

@@ -14,5 +14,6 @@ install:
 script:
   - py.test --doctest-modules --cov=doi src tests
   - mypy src/ tests
+  - flake8 src/ tests
 after_success:
   - coveralls

src/doi/__init__.py

@@ -43,7 +43,7 @@ def validate_doi(doi: str) -> Optional[str]:
     :param doi: Identifier.
     :returns: The URL assigned to the DOI or ``None``.
     """
-    from urllib.error import HTTPError, URLError
+    from urllib.error import HTTPError
     import urllib.request
     import urllib.parse
     import json

tests/test_doi.py

@@ -66,7 +66,7 @@ def test_find_doi_in_line() -> None:
         ('http://.scitation.org/doi/10.1063/1.mart(88)1498?asdfwer',
             '10.1063/1.mart(88)1498'),
         ('@ibook{doi:10.1002/9780470125915.ch2,', '10.1002/9780470125915.ch2'),
-        ('<rdf:Description rdf:about="" xmlns:dc="http://purl.org/dc/elements/1'
+        ('<rdf:Description rdf:about="" xmlns:dc="http://purl.org/dc/elements'
          '.1/"><dc:format>application/pdf</dc:format><dc:identifier>'
          'doi:10.1063/1.5079474</dc:identifier></rdf:Description>',
             '10.1063/1.5079474'),