SSL connection handshake charset is always set to utf-8

[alviezhang]

Describe the bug

Last year, I executed a SQL by using aiomysql, want to insert a string with emoji into a MySQL table, like

INSERT INTO xxx values("sentence with 😄")

It reported the following error

E       pymysql.err.DataError: (1366, "Incorrect string value: '\\xF0\\x9F\\x98\\x84' for column 'a' at row 1")

I double-checked the table charset

CREATE TABLE xxx (a text) DEFAULT CHARACTER SET="utf8mb4")

and the connection charset

aiomysql.create_pool(..., charset='utf8mb4')

everything was looks well.

So I checked the real character set of the aiomysql client connecting to the MySQL server

 await cursor.execute("SHOW SESSION VARIABLES LIKE 'character\_set\_%';")
 print(await cursor.fetchone())
> 

and the output was

('character_set_client', 'utf8')

I also wrote the same code with PyMySQL, which was successfully executed, so this is an aiomysql only issue.

After doing a little investigation, I found the source of this issue:

aiomysql/aiomysql/connection.py

Line 738 in 1e474e6

data = struct.pack('<IIB', self.client_flag, 16777216, 33)

I don't know why the encoding is set to a constant value of 33, which means that utf8 is fixed as the encoding
https://github.com/PyMySQL/PyMySQL/blob/main/pymysql/charset.py#L95

To Reproduce

Having table

CREATE TABLE test_string_with_emoji (a text) DEFAULT CHARACTER SET="utf8mb4";

and execute the following code

test_value = "I am a test string with emoji 😄"

await cursor.execute("INSERT INTO test_string_with_emoji (a) VALUES (%s)", test_value)

Expected behavior

Insert successfully

Logs/tracebacks

def raise_mysql_exception(data):
        errno = struct.unpack("<h", data[1:3])[0]
        errval = data[9:].decode("utf-8", "replace")
        errorclass = error_map.get(errno)
        if errorclass is None:
            errorclass = InternalError if errno < 1000 else OperationalError
>       raise errorclass(errno, errval)
E       pymysql.err.DataError: (1366, "Incorrect string value: '\\xF0\\x9F\\x98\\x84' for column 'a' at row 1")

.venv/lib/python3.10/site-packages/pymysql/err.py:143: DataError

### Python Version

```console
$ python --version
3.10.2

aiomysql Version

$ python -m pip show aiomysql
0.1.0

PyMySQL Version

$ python -m pip show PyMySQL
1.0.2

SQLAlchemy Version

$ python -m pip show sqlalchemy
1.3.24

OS

Arch Linux

Database type and version

SELECT VERSION();
+-----------+
| VERSION() |
+-----------+
| 5.7.37    |
+-----------+
1 row in set (0.00 sec)

Additional context

No response

Code of Conduct

• I agree to follow the aio-libs Code of Conduct

[alviezhang]

I submitted a PR to fix this issue #776