Added solution of problem 1581

Author: kurpenok

src/solutions/s1581_customer_who_visited_but_did_not_make_any_transactions.py

@@ -0,0 +1,10 @@
+import pandas as pd
+
+
+def find_customers(visits: pd.DataFrame, transactions: pd.DataFrame) -> pd.DataFrame:
+    df = visits.merge(transactions, how="left")
+    return (
+        df[df["transaction_id"].isna()]
+        .groupby(["customer_id"], as_index=False)
+        .agg(count_no_trans=("visit_id", "nunique"))
+    )