add type annotations and pylint configuration

This MR includes

• type annotations (compatible with both Python 2 and 3) for the module
• changes needed to fix bugs highlighted by the type annotations
• changes needed to fix bugs highlighted by pylint
• a pylint config to silence more annoying warnings that we are unlikely to fix

The final code is clean with mypy (type checking) and pylint3 (current upstream release, since the Debian package is not currently installable).

lib/debian/tests/test_deb822.py

@@ -28,14 +28,16 @@ import sys
 import tempfile
 import unittest
 import warnings
-try:
+
+import six
+
+if six.PY3:
+    from io import BytesIO, StringIO
+else:
     from StringIO import StringIO
     BytesIO = StringIO
-except ImportError:
-    from io import BytesIO, StringIO
 
-import apt_pkg
-import six
+import apt_pkg   #type: ignore
 
 from debian import deb822
 
@@ -374,12 +376,12 @@ class TestDeb822(unittest.TestCase):
             self.assertEqual(v, deb822_[k])
         self.assertEqual(deb822_, dict_)
 
+    @staticmethod
     def gen_random_string(length=20):
         from random import choice
         import string
         chars = string.ascii_letters + string.digits
         return ''.join([choice(chars) for i in range(length)])
-    gen_random_string = staticmethod(gen_random_string)
 
     def deb822_from_format_string(self, string, dict_=PARSED_PACKAGE, cls=deb822.Deb822):
         """Construct a Deb822 object by formatting string with % dict.
@@ -856,22 +858,18 @@ Description: python modules to work with Debian-related data formats
         # Avoid spitting out the encoding warning during testing.
         warnings.filterwarnings(action='ignore', category=UnicodeWarning)
 
-        filename = 'test_Sources.mixed_encoding'
-        # TODO: With Python >= 2.7, this might be better written as:
-        #   with open(filename, 'rb') as f1, open(filename, 'rb') as f2:
-        f1 = open(find_test_file(filename), 'rb')
-        f2 = open(find_test_file(filename), 'rb')
-        for paragraphs in [deb822.Sources.iter_paragraphs(f1),
-                           deb822.Sources.iter_paragraphs(f2,
-                                                          use_apt_pkg=False)]:
-            p1 = next(paragraphs)
-            self.assertEqual(p1['maintainer'],
-                             six.u('Adeodato Sim\xf3 <dato@net.com.org.es>'))
-            p2 = next(paragraphs)
-            self.assertEqual(p2['uploaders'],
-                             six.u('Frank K\xfcster <frank@debian.org>'))
-        f2.close()
-        f1.close()
+        filename = find_test_file('test_Sources.mixed_encoding')
+        with open(filename, 'rb') as f1, open(filename, 'rb') as f2:
+            for paragraphs in [
+                        deb822.Sources.iter_paragraphs(f1),
+                        deb822.Sources.iter_paragraphs(f2, use_apt_pkg=False)
+                    ]:
+                p1 = next(paragraphs)
+                self.assertEqual(p1['maintainer'],
+                                six.u('Adeodato Sim\xf3 <dato@net.com.org.es>'))
+                p2 = next(paragraphs)
+                self.assertEqual(p2['uploaders'],
+                                six.u('Frank K\xfcster <frank@debian.org>'))
 
     def test_dump_text_mode(self):
         d = deb822.Deb822(CHANGES_FILE.splitlines())
@@ -1304,9 +1302,9 @@ def _no_space(s):
 
 class RestrictedWrapperTest(unittest.TestCase):
     class Wrapper(deb822.RestrictedWrapper):
-        restricted_field = deb822.RestrictedField('Restricted-Field')
-        required_field = deb822.RestrictedField('Required-Field', allow_none=False)
-        space_separated = deb822.RestrictedField(
+        restricted_field = deb822.RestrictedField('Restricted-Field')  # type: ignore
+        required_field = deb822.RestrictedField('Required-Field', allow_none=False)   # type: ignore
+        space_separated = deb822.RestrictedField(    # type: ignore
                 'Space-Separated',
                 from_str=lambda s: tuple((s or '').split()),
                 to_str=lambda seq: ' '.join(_no_space(s) for s in seq) or None)