fix: query copying

[henadzit]

Overview

This fixes the issue where the copies of a query might share a state - QueryBuilder._on_conflict_fields. This PR should fix tortoise/tortoise-orm#1722 in tortoise-orm.

Issue

from pypika.dialects.postgresql import PostgreSQLQuery
from pypika.queries import Table


table_abc = Table("abc")
base_query = PostgreSQLQuery.into(table_abc).insert(1)

query1 = base_query.on_conflict(table_abc.id).do_nothing()
print("query1 before the issue:", query1)

# this introduces an issue
query2 = base_query.on_conflict(table_abc.id).do_nothing()
print("query1:", query1)
print("query2:", query2)

This will print

query before the issue INSERT INTO "abc" VALUES (1) ON CONFLICT ("id") DO NOTHING
query1 INSERT INTO "abc" VALUES (1) ON CONFLICT ("id", "id") DO NOTHING
query2 INSERT INTO "abc" VALUES (1) ON CONFLICT ("id", "id") DO NOTHING

Please note ... ON CONFLICT ("id", "id") ... after the second query was created!

Here's the code in tortoise-orm which is affected by this issue
https://github.com/tortoise/tortoise-orm/blob/0ddf8d327949be3ad812c41e4215c85b997214b9/tortoise/queryset.py#L1882

[waketzheng]

@henadzit Cloud you help to review this PR: #10