Use psycopg3 if psycopg2 is not installed.

Fixes #3036

CHANGELOG.md

@@ -21,6 +21,7 @@ https://github.com/coleifer/peewee/releases
 * Better handling of duplicated column names in cursor wrapper implementations.
 * Improve performance of ModelCursorWrapper when reconstructing model instance
   graphs after multi-table selects.
+* If only psycopg3 is installed, use it by default (#3036)
 
 [View commits](https://github.com/coleifer/peewee/compare/4.0.2...master)
 

peewee.py

@@ -4138,7 +4138,7 @@ class PostgresqlDatabase(Database):
         self._encoding = encoding
 
         prefer_psycopg3 = kwargs.pop('prefer_psycopg3', False)
-        if psycopg is not None and prefer_psycopg3:
+        if psycopg is not None and (prefer_psycopg3 or psycopg2 is None):
             self._adapter = self.psycopg3_adapter()
         else:
             self._adapter = self.psycopg2_adapter()