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

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.