global: use Python feature detection in _compat.py

* BETTER Python porting best practice use feature detection instead
  of version detection to save an import and pass both PyLint
  and Flake8 tests with neither 'pragma' nor 'noqa'.

dictdiffer/_compat.py

@@ -8,23 +8,22 @@
 
 """Python compatibility definitions."""
 
-import sys
-
-if sys.version_info[0] == 3:  # pragma: no cover (Python 2/3 specific code)
+try:
+    PY2 = True
+    string_types = basestring,
+    text_type = unicode
+    num_types = int, long, float
+except NameError:
+    PY2 = False
     string_types = str,
     text_type = str
     num_types = int, float
-    PY2 = False
 
+if PY2:
+    from collections import MutableMapping, MutableSet, MutableSequence
+    from itertools import izip_longest as _zip_longest
+else:
     from collections.abc import MutableMapping, MutableSet, MutableSequence
     from itertools import zip_longest as _zip_longest
-    izip_longest = _zip_longest
-else:  # pragma: no cover (Python 2/3 specific code)
-    string_types = basestring,
-    text_type = unicode
-    num_types = int, long, float
-    PY2 = True
 
-    from collections import MutableMapping, MutableSet, MutableSequence
-    from itertools import izip_longest as _zip_longest
-    izip_longest = _zip_longest
+izip_longest = _zip_longest