.pgpass file support handles backslash escapes incorrectly; should de-escape values

Nobody has claimed this yet.

https://www.postgresql.org/docs/current/libpq-pgpass.html isn't fully explicit about how to handle backslash escapes in the `.pgpass` file, however it's reasonably clear from the phrasing that backslashes serve only to escape backslashes and colons and therefore aren't considered part of the value they're used in. E.g., a `.pgpass` row of …

```
*:*:test\:db:test\\:password from pgpass with escapes
```

conveys:

- a database name of `test:db`, not `test\:db`
- a user name of `test\`, not `test\\`

I ran into this problem because I have a password of `4:K1…`, encoded as `4\:K1…`, and it works with `psql` but not `asyncpg`. Once I remove the backslash from `.pgpass`, it starts working with `asyncpg` but failing with `psql`.

I would've provided a failing test and even a code change, but I can't get `python setup.py test` or `pip install -e .[test]` to work in a run-of-the-mill Python 3.12 virtualenv on my run-of-the-mill MacOS.

Start by resolving the Python 3.12 test setup issue reported for `python setup.py test` or `pip install -e .[test]`, then locate asyncpg's `.pgpass` handling. Add coverage for escaped colons and backslashes, and consider the issue done when those values are de-escaped consistently with `psql`.

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