Don't require all Python database modules to be installed

We support multiple database backends. Don't require Python modules for
all backends to be installed.
Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>

aurweb/db.py

-import mysql.connector
-import sqlite3
+try:
+    import mysql.connector
+except ImportError:
+    pass
+
+try:
+    import sqlite3
+except ImportError:
+    pass
 
 import aurweb.config