Fix typing with newer types-psycopg2

Since `connection` is no longer exported from psycopg2 stubs, it's
not possible to specify explicit type. Howerver, mypy does the right
job to guess the type, so we can omit explicit annotation.

repologyapp/db.py

@@ -34,8 +34,6 @@
 
 
 class Database:
-    _db: 'psycopg2.connection'
-
     def __init__(self, dsn: str, querymgr: QueryManager, readonly: bool = True, autocommit: bool = False, application_name: str | None = None) -> None:
         self._db = psycopg2.connect(dsn, application_name=application_name)
         self._db.set_session(readonly=readonly, autocommit=autocommit)