Forbid build metadata ordering (See #18)

SemVer 2.0.0 states that "Build metadata SHOULD be ignored when
determining version precedence".

This means that, when comparing ``0.1.0+1`` to ``0.1.0+bcd``::

    >>> Version('0.1.0+1') == Version('0.1.0+bcd')
    False
    >>> Version('0.1.0+1') != Version('0.1.0+bcd')
    True
    >>> Version('0.1.0+1') < Version('0.1.0+bcd')
    False
    >>> Version('0.1.0+1') > Version('0.1.0+bcd')
    False
    >>> Version('0.1.0+1') <= Version('0.1.0+bcd')
    False
    >>> Version('0.1.0+1') >= Version('0.1.0+bcd')
    False
    >>> compare(Version('0.1.0+1'), Version('0.1.0+bcd'))
    NotImplemented

This change also has the following effects:
- When including build metadata in a ``Spec``, the only valid options
  are ``Spec('==0.1.0+sth')`` and ``Spec('!=0.1.0+sth')``
- The meaning of ``Spec('==0.1.0+')`` is now "Only version 0.1.0 without
  build metadata"
- ``Spec('==0.1.0')`` now matches ``Version('0.1.0+anything')``

1 files changed, 10 insertions, 13 deletions

diff --git a/semantic_version/compat.py b/semantic_version/compat.py
index bea6f67..4dd60fe 100644
--- a/semantic_version/compat.py
+++ b/semantic_version/compat.py
@@ -2,17 +2,14 @@
 # Copyright (c) 2012-2014 The python-semanticversion project
 # This code is distributed under the two-clause BSD License.
 
-import sys
 
-is_python2 = (sys.version_info[0] == 2)
-
-if is_python2:  # pragma: no cover
-    base_cmp = cmp
-else:  # pragma: no cover
-    def base_cmp(x, y):
-        if x < y:
-            return -1
-        elif x > y:
-            return 1
-        else:
-            return 0
+def base_cmp(x, y):
+    if x == y:
+        return 0
+    elif x > y:
+        return 1
+    elif x < y:
+        return -1
+    else:
+        # Fix Py2's behavior: cmp(x, y) returns -1 for unorderable types
+        return NotImplemented