[MNT] Add tests for db module

tests/test_db.py

@@ -0,0 +1,20 @@
+from mendeleev.db import get_engine
+from sqlalchemy import text as sql_text
+import pandas as pd
+
+
+def test_read_sql_table():
+    engine = get_engine()
+    query = "select * from elements"
+    with engine.begin() as conn:
+        df = pd.read_sql_query(sql=sql_text(query), con=conn)
+    assert df is not None
+
+
+def test_read_sql_table_old():
+    engine = get_engine()
+    query = "select * from elements"
+    # with engine.begin() as conn:
+    conn = engine.connect()
+    df = pd.read_sql_query(sql=query, con=conn)
+    assert df is not None