Upgrade mautrix to v0.19.15

requirements.txt

@@ -4,7 +4,7 @@ commonmark>=0.8,<0.10
 aiohttp>=3,<4
 yarl>=1,<2
 attrs>=20.1
-mautrix>=0.19.14,<0.20
+mautrix>=0.19.15,<0.20
 asyncpg>=0.20,<0.28
 pycryptodome>=3,<4
 paho-mqtt>=1.5,<2