move tantivy-py installation to be separate from wheel (#97)

pypi does not allow packages to be uploaded that has a direct reference

for now we'll just ask the user to install tantivy separately

---------

Co-authored-by: Chang She <chang@lancedb.com>

python/tests/test_fts.py

@@ -13,13 +13,13 @@
 import os
 import random
 
+import lancedb.fts
 import numpy as np
 import pandas as pd
 import pytest
 import tantivy
 
 import lancedb as ldb
-import lancedb.fts
 
 
 @pytest.fixture