Exception when attempting to fetch SSL info

Thanks for creating asyncpg! It's dramatically improved the performance of my open-source web application (Runestone Academy, a free interactive e-book).

To reproduce this bug, simply start asyncpg as a non-root user (one without permission to access /root).

• asyncpg version: 0.25
• PostgreSQL version: 12.7
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?: I use AWS RDS; haven't tested locally
• Python version: 3.9.1
• Platform: Debian GNU/Linux 11 (bullseye)
• Do you use pgbouncer?: No
• Did you install asyncpg with pip?: Yes
• If you built asyncpg locally, which version of Cython did you use?: N/A
• Can the issue be reproduced under both asyncio and
  uvloop?: I only use asyncio

I run asyncpg as a non-root user for improved security; this user lacks root access. During startup in asyncpg v. 0.25, I see the error like this:

  File "/srv/web2py/applications/runestone/.venv/lib/python3.9/site-packages/asyncpg/connection.py", line 2085, in connect
    return await connect_utils._connect(
  File "/srv/web2py/applications/runestone/.venv/lib/python3.9/site-packages/asyncpg/connect_utils.py", line 874, in _connect
  addrs, params, config = _parse_connect_arguments(timeout=timeout, **kwargs)
  File "/srv/web2py/applications/runestone/.venv/lib/python3.9/site-packages/asyncpg/connect_utils.py", line 640, in _parse_connect_arguments
  addrs, params = _parse_connect_dsn_and_args(
  File "/srv/web2py/applications/runestone/.venv/lib/python3.9/site-packages/asyncpg/connect_utils.py", line 543, in _parse_connect_dsn_and_args
    if not sslkey.exists():
  File "/usr/local/lib/python3.9/pathlib.py", line 1424, in exists
    self.stat()
  File "/usr/local/lib/python3.9/pathlib.py", line 1232, in stat
    return self._accessor.stat(self)
PermissionError: [Errno 13] Permission denied

In connect_utils.py line 543, asyncpg checks if a root-owned file exists. Unfortunately, a non-root user gets a permission denied exception instead of a False return value from exists(). It looks like wrapping this in a try/except would fix this bug. (It looks like a later exception needs PermissionError added to it.)

For me, reverting to asyncpg v. 0.24 causes my code to run without problems.

从 connect_utils.py 约第 543 行开始，这里会在解析连接参数时检查 SSL 密钥是否存在。以无法访问 /root 的非 root 用户重现该连接，然后验证 SSL 查找不再引发 PermissionError，并且连接会像使用 asyncpg 0.24 时一样继续进行。