PostgreSQL driver, tests against DB backends, and general drivers cleanup (#2723)

* PostgreSQL driver and general drivers cleanup

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Make tests pass

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Add black --target-version flag in make.bat

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Rewrite postgres driver

Most of the logic is now in PL/pgSQL.

This completely avoids the use of Python f-strings to format identifiers into queries. Although an SQL-injection attack would have been impossible anyway (only the owner would have ever had the ability to do that), using PostgreSQL's format() is more reliable for unusual identifiers. Performance-wise, I'm not sure whether this is an improvement, but I highly doubt that it's worse.

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Reformat

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Fix PostgresDriver.delete_all_data()

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Clean up PL/pgSQL code

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* More PL/pgSQL cleanup

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* PL/pgSQL function optimisations

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Ensure compatibility with PostgreSQL 10 and below

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* More/better docstrings for PG functions

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Fix typo in docstring

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Return correct value on toggle()

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Use composite type for PG function parameters

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Fix JSON driver's Config.clear_all()

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Correct description for Mongo tox recipe

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Fix linting errors

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Update dep specification after merging bumpdeps

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Add towncrier entries

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Update from merge

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Mention [postgres] extra in install docs

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Support more connection options and use better defaults

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Actually pass PG env vars in tox

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Replace event trigger with manual DELETE queries

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

redbot/core/errors.py

@@ -1,5 +1,4 @@
 import importlib.machinery
-from typing import Optional
 
 import discord
 
@@ -49,3 +48,37 @@ class BalanceTooHigh(BankError, OverflowError):
         return _("{user}'s balance cannot rise above {max:,} {currency}.").format(
             user=self.user, max=self.max_balance, currency=self.currency_name
         )
+
+
+class MissingExtraRequirements(RedError):
+    """Raised when an extra requirement is missing but required."""
+
+
+class ConfigError(RedError):
+    """Error in a Config operation."""
+
+
+class StoredTypeError(ConfigError, TypeError):
+    """A TypeError pertaining to stored Config data.
+
+    This error may arise when, for example, trying to increment a value
+    which is not a number, or trying to toggle a value which is not a
+    boolean.
+    """
+
+
+class CannotSetSubfield(StoredTypeError):
+    """Tried to set sub-field of an invalid data structure.
+
+    This would occur in the following example::
+
+        >>> import asyncio
+        >>> from redbot.core import Config
+        >>> config = Config.get_conf(None, 1234, cog_name="Example")
+        >>> async def example():
+        ...     await config.foo.set(True)
+        ...     await config.set_raw("foo", "bar", False)  # Should raise here
+        ...
+        >>> asyncio.run(example())
+
+    """