Fix AttributeError on self.ssl in sha256_password auth branch

[shychee]

Summary

• Fix missed self.ssl → self._ssl_context rename in the sha256_password authentication branch of _request_authentication
• PR Fix ssl handshake error with mysql server 8.0.34+ #102 renamed self.ssl to self._ssl_context everywhere except line 825, causing AttributeError when connecting to databases using sha256_password plugin
• Update CHANGELOG

Change

- if self.ssl and self.server_capabilities & SSL:
+ if self._ssl_context and self.server_capabilities & SSL:

How to Reproduce

1. Connect to a MySQL-compatible database (e.g. Huawei GaussDB) using sha256_password auth plugin
2. Without SSL (ssl=None)
3. Raises: AttributeError: 'Connection' object has no attribute 'ssl'

Fixes #147

[shychee]

@long2ice This is a one-line fix for a bug introduced by PR #102 — self.ssl was missed in the sha256_password branch.
Could you please take a look? Thanks!