add indexes to improve the speed of calc.py

fixes #33

apps/rates/migrations/0002_exchangerate_exchangerat_crypto__8e5f68_idx.py

@@ -0,0 +1,17 @@
+# Generated by Django 3.2.5 on 2021-08-25 15:58
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('rates', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AddIndex(
+            model_name='exchangerate',
+            index=models.Index(fields=['crypto_currency', 'fiat_currency', 'datetime_valid'], name='exchangerat_crypto__8e5f68_idx'),
+        ),
+    ]