Fix create/drop indexes in every migration (#377)

* Add `__eq__` method for `Index`instances

* tests: add Index test case

* refactor: compare index instances before set hash and eq func to class

* fix: sort fields when generating index hash

* docs: update changlog

* fix style issue

* refactor: use CustomIndex instead of postgres special HashIndex

* Check tortoise version before patch Index

* Add comment

* Add comment for why > work

---------

Co-authored-by: dbf <somnium@riseup.net>

tests/old_models.py

@@ -2,6 +2,9 @@ import datetime
 from enum import IntEnum
 
 from tortoise import Model, fields
+from tortoise.indexes import Index
+
+from tests.indexes import CustomIndex
 
 
 class ProductType(IntEnum):
@@ -31,6 +34,9 @@ class User(Model):
     intro = fields.TextField(default="")
     longitude = fields.DecimalField(max_digits=12, decimal_places=9)
 
+    class Meta:
+        indexes = [Index(fields=("username", "is_active")), CustomIndex(fields=("is_superuser",))]
+
 
 class Email(Model):
     email = fields.CharField(max_length=200)