Use environment and pgpass to connect to PostgreSQL

[Natureshadow]

Closes #384.

Instead of building a command-line that might be subject to a row of issues (the command-line used before carried the database password in plaintext, making it available to everyone with read access to the system, we now build a copy of the environment with all the libpq variables that are needed prefilled. The password is written into a .pgpass file as suggested by the PostgreSQL docs.

This also makes the call more compatible with various setups. For instance, a hostname is not mandatory — pg_dump can connect through the UNIX socket in /var/run as well.

In order to make all that re-usable and easy to read and understand, I decided to pack the environment generation and .pgpass file handling in a context manager. Someone else (or me, later on, if I get bored) might want to copy it for MySQL and MongoDB.

[Archmonger]

The master branch needs to be re-merged into this PR

Also, we only support Django > 3.2 now. Would you like to rewrite this to utilize settings_to_cmd_args_env?