Add `Version.precedence_key`.

This will be used in `sort(..., key=lambda v: v.precedence_key)`.
Remove previous comparison/ordering implementation.

The current implementation relies on a 4-tuple:
- major, minor, patch (as integers) - natural order matches precedence
  rules
- a tuple of identifiers for the prerelease component.

The identifiers for the prerelease components are based on:
- A `NumericIdentifier` class, that compares number using their natural
  order, and always compares lower than non-numeric identifiers
- A `AlphaIdentifier` class for non-numeric identifiers; it compares
  versions using ASCII ordering.
- A `MaxIdentifier` class, that compares higher to any other identifier;
  used to ensure that a non-prerelease version is greater than any of
  its prereleases.

1 files changed, 2 insertions, 2 deletions

diff --git a/tests/test_parsing.py b/tests/test_parsing.py
index 2d612a1..0da679f 100755
--- a/tests/test_parsing.py
+++ b/tests/test_parsing.py
@@ -120,9 +120,9 @@ class ComparisonTestCase(unittest.TestCase):
                     self.assertFalse(v1 == v2, "%r == %r" % (v1, v2))
                     self.assertTrue(v1 != v2, "%r !!= %r" % (v1, v2))
                     self.assertFalse(v1 < v2, "%r !< %r" % (v1, v2))
-                    self.assertFalse(v1 <= v2, "%r !<= %r" % (v1, v2))
+                    self.assertTrue(v1 <= v2, "%r !<= %r" % (v1, v2))
                     self.assertFalse(v2 > v1, "%r !> %r" % (v2, v1))
-                    self.assertFalse(v2 >= v1, "%r !>= %r" % (v2, v1))
+                    self.assertTrue(v2 >= v1, "%r !>= %r" % (v2, v1))
 
 
 if __name__ == '__main__':  # pragma: no cover