Unable to use f-string in SqlAlchemy ORM

还没有人认领这个 Issue。

* **asyncpg version**: 0.23.0
* **PostgreSQL version**: 12
* **Python version**: 3.9.4
* **Platform**: OSX
* **Do you use pgbouncer?**: No
* **Did you install asyncpg with pip?**: pipenv
* **If you built asyncpg locally, which version of Cython did you use?**: N/A
* **Can the issue be reproduced under both asyncio and
[uvloop](https://github.com/magicstack/uvloop)?**: N/A

I'm trying to build a SQL statement with LIKE expression and a pattern but I'm getting an error.

Here is what I had in the code originally:
```
statement = select(Customer).options(sqlalchemy.orm.joinedload(Customer.transactions)).where(Customer.l_name.ilike(name))
```
and this worked great.

But if I want to use this statement with a % pattern like this:

```
statement = f'select(Customer).options(sqlalchemy.orm.joinedload(Customer.transactions)).where(Customer.l_name.ilike("%{name}%"))'
```

I get this error:

```
sqlalchemy.exc.ProgrammingError: (sqlalchemy.dialects.postgresql.asyncpg.ProgrammingError) : syntax error at or near "("
[SQL: select(Customer).options(sqlalchemy.orm.joinedload(Customer.transactions)).where(Customer.l_name.ilike("%%turing%%"))]
```

It does substitute the variable with a parameter but for some reason it adds an extra pair of percent signs and crashes.
How can I solve this problem?

未指定任何 asyncpg 文件、测试或入口点。先从提供的 SQLAlchemy 表达式和生成的 SQL 开始，然后验证该行为是否能在 asyncpg 中复现，或者是否源于将 ORM 语句构造成字符串；根据列出的 PostgreSQL 和 asyncpg 版本确认结果。

由索引模型根据 Issue 内容生成。