mysql: bump sqlalchemy.url MariaDB/MySQL charset to to 'utf8mb4' to get full UTF-8 support

The change in 210e76d69b62 only changed character_set_database, as shown by output after:

--- a/kallithea/model/base.py
+++ b/kallithea/model/base.py
@@ -46,3 +46,8 @@ def init_model(engine):
engine_str = obfuscate_url_pw(str(engine.url))
log.info("initializing db for %s", engine_str)
meta.Base.metadata.bind = engine
+
+ meta.Session.configure(bind=engine)
+ for a, b in meta.Session().execute('''show variables''').fetchall():
+ if 'character_set_' in a:
+ print(a, repr(b))

Before, with charset=utf8, the utf8mb3 charset was used all the way through the stack:

[kallithea.model.base] initializing db for mysql://kallithea-test:XXXXX@localhost/kallithea-test?charset=utf8
character_set_client 'utf8'
character_set_connection 'utf8'
character_set_database 'utf8mb4'
character_set_filesystem 'binary'
character_set_results 'utf8'
character_set_server 'latin1'
character_set_system 'utf8'

With explicit charset=utf8mb4:

[kallithea.model.base] initializing db for mysql://kallithea-test:XXXXX@localhost/kallithea-test?charset=utf8mb4
character_set_client 'utf8mb4'
character_set_connection 'utf8mb4'
character_set_database 'utf8mb4'
character_set_filesystem 'binary'
character_set_results 'utf8mb4'
character_set_server 'latin1'
character_set_system 'utf8'