Upgrade pip package requirements

Release v0.7.3 with security advice about PyMySQL 1.1.1

fixes for webhook JWT token generation

introduce WEBHOOK_USE_JWT env var to make token type better configurable
prepare for v0.7.2 release

WEBHOOK_URL can now contain any key of the passed JSON object as plac…

…eholder

prepare for v0.7.1 release

Improved Python code style by fixing all Flake8 linting warnings

Database cleanup job now also purges old messages, introduced `MESSAG…

…E_RETENTION` env var

get ready for v0.6.1 release

Introduce DBUtils PooledDB db connection pooling (#3)

* refactored database.db to app.db to simplify project structure

* implement db connection pooling with DBUtils PooledDB (pooled_db)

* minor fix in Handler: only close db connection if it was initialized

* minor fix for previous commit

* start explicit transaction with db.begin() in Cleanup, as required by DBUtils PooledDB

* moved logger and db cleanup code into destructor of Handler
made DBUtils PooledDB params configurable via .env vars

* added db connection pooling configuration notes to README
getting ready for v0.6.0 release

Prefix all log messages with msgid (Postfix queue_id) if available

get ready to publish initial release v0.5.0