Removed Python2 support

Closes: #6890

Some remarks,

- `# coding ... utf-8` statements are not needed
- isdigit on strings instead of a try-catch.
- Default opening mode is read, so we can do `open()` without the `'r'` everywhere
- Removed inheritinng from `object` class, it isn't necessary in python3.
- `super(ClassName, self)` can now be replaced with `super()`
- Use of itertools and `chain` on a couple places dealing with sets.
- Used the operator module instead of lambdas when warranted
    `itemgetter(0)` instead of `lambda x: x[0]`
    `attrgetter('name')` instead of `lambda x: x.name`
- `sorted` returns a list, so no need to use `list(sorted(...))`
- Removed `dict()` using dictionary comprehensions whenever possible
- Attempted to remove python3.2 support

Signed-off-by: alexrecuenco <alejandrogonzalezrecuenco@gmail.com>

tests/unit/split_buffer_test.py

@@ -36,7 +36,7 @@ class SplitBufferTest(unittest.TestCase):
         self.assert_produces(reader, ['abc\n', 'd'])
 
     def test_preserves_unicode_sequences_within_lines(self):
-        string = u"a\u2022c\n"
+        string = "a\u2022c\n"
 
         def reader():
             yield string.encode('utf-8')