feat(290): Cache previous/next/etc. in VersionClassBase

Cache some properties in VersionClassBase as it won't really change
and can improve performance significantly by avoiding extra queries

Tested this in my own application and also in a shell:
>>> class A:
...     @cached_property
...     def abc(self):
...         print("test")
...         return 1
...
>>> a = A()
>>> a.abc
test
1
>>> a.abc
1

setup.py

@@ -64,7 +64,8 @@ def get_version():
     platforms='any',
     install_requires=[
         'SQLAlchemy>=1.0.8',
-        'SQLAlchemy-Utils>=0.30.12'
+        'SQLAlchemy-Utils>=0.30.12',
+        'cached-property'
     ],
     extras_require=extras_require,
     classifiers=[

sqlalchemy_continuum/version.py

@@ -1,11 +1,12 @@
 import sqlalchemy as sa
+from cached_property import cached_property
 
 from .reverter import Reverter
 from .utils import get_versioning_manager, is_internal_column, parent_class
 
 
 class VersionClassBase(object):
-    @property
+    @cached_property
     def previous(self):
         """
         Returns the previous version relative to this version in the version
@@ -18,7 +19,7 @@ def previous(self):
             .previous(self)
         )
 
-    @property
+    @cached_property
     def next(self):
         """
         Returns the next version relative to this version in the version
@@ -31,7 +32,7 @@ def next(self):
             .next(self)
         )
 
-    @property
+    @cached_property
     def index(self):
         """
         Return the index of this version in the version history.