Refactor spec/version matching.

Instead of choosing the comparison on each `.match()` call, the
expression is converted to a combination of `Range()` expressions
(simple comparison to a semver-compliant `Version`).

`Range()` objects can be combined with `And` and `Or` through the
`AnyOf` and `AllOf` clauses (sticking to Python's naming scheme).

Some specific flags have been provided to those range, allowing users to
subtly alter the matching behaviour - thus accomodating different
versioning schemes:
- `<0.1.2` won't match `0.1.2-rc1`, unless the prerelease_policy flag is
  set to either `always` or `same-patch`
- `<0.1.2` will match `0.1.1-rc1`, unless the `prerelease_policy` flag
  is set to `same-patch`
- `==0.1.2` will always match `0.1.2+build44`, unless the `build_policy`
  is set to `strict`.

The `Spec` item has been updated, alongside `SpecItem`.
Those objects keep the original expression as attributes, but don't use
them for comparisons.

1 files changed, 13 insertions, 2 deletions

diff --git a/semantic_version/django_fields.py b/semantic_version/django_fields.py
index 2e9be69..1af5bf5 100644
--- a/semantic_version/django_fields.py
+++ b/semantic_version/django_fields.py
@@ -74,10 +74,21 @@ class SpecField(SemVerField):
     }
     description = _("Version specification list")
 
+    def __init__(self, *args, **kwargs):
+        self.syntax = kwargs.pop('syntax', base.DEFAULT_SYNTAX)
+        super().__init__(*args, **kwargs)
+
+    def deconstruct(self):
+        """Handle django.db.migrations."""
+        name, path, args, kwargs = super().deconstruct()
+        if self.syntax != base.DEFAULT_SYNTAX:
+            kwargs['syntax'] = self.syntax
+        return name, path, args, kwargs
+
     def to_python(self, value):
         """Converts any value to a base.Spec field."""
         if value is None or value == '':
             return value
-        if isinstance(value, base.Spec):
+        if isinstance(value, base.BaseSpec):
             return value
-        return base.Spec(value)
+        return base.BaseSpec.parse(value, syntax=self.syntax)