use only s3 in boto3-stubs and update mypy

Newer version of mypy fixes buggy error when trying to update only boto3 stubs.
However it brings new checks and starts to yell when we index into
cusror.fetchone without checking for None first. So this introduces a wrapper
to simplify quering for scalar values. I tried to use cursor_factory connection
argument but without success. There can be a better way to do that,
but this looks the simplest

test_runner/performance/test_random_writes.py

@@ -9,6 +9,8 @@ import psycopg2.extras
 import random
 import time
 
+from fixtures.utils import query_scalar
+
 
 # This is a clear-box test that demonstrates the worst case scenario for the
 # "1 segment per layer" implementation of the pageserver. It writes to random
@@ -59,9 +61,7 @@ def test_random_writes(neon_with_baseline: PgCompare):
                     rows_inserted += rows_to_insert
 
             # Get table size (can't be predicted because padding and alignment)
-            cur.execute("SELECT pg_relation_size('Big');")
-            row = cur.fetchone()
-            table_size = row[0]
+            table_size = query_scalar(cur, "SELECT pg_relation_size('Big')")
             env.zenbenchmark.record("table_size", table_size, 'bytes', MetricReport.TEST_PARAM)
 
             # Decide how much to write, based on knowledge of pageserver implementation.