Handle logging queries encoded as bytes under PostgreSQL

[Lucidiot]

Description

Unlike SQLite, psycopg supports queries encoded as bytes, and uses this in its own methods such as in execute_values. But the SQL logging only expects strings or Composables. This was causing an AttributeError such as this:

Traceback (most recent call last):
[snip]
  File "/home/lucidiot/.virtualenvs/tmp/lib/python3.10/site-packages/psycopg2/extras.py", line 1270, in execute_values
    cur.execute(b''.join(parts))
  File "/home/lucidiot/.virtualenvs/tmp/lib/python3.10/site-packages/debug_toolbar/panels/sql/tracking.py", line 246, in execute
    return self._record(super().execute, sql, params)
  File "/home/lucidiot/.virtualenvs/tmp/lib/python3.10/site-packages/debug_toolbar/panels/sql/tracking.py", line 181, in _record
    sql = sql.as_string(conn)
AttributeError: 'bytes' object has no attribute 'as_string'

This adds an extra condition to decode a bytes instance with .decode() instead of .as_string().

Fixes #987

Checklist:

• I have added the relevant tests for this change.
• I have added an item to the Pending section of docs/changes.rst.

[matthiask]

Thank you!