Bump version: 4.2.0

[psrok1]

feature: karton logs command

A new CLI command to follow logs from a specified kartons (#111). Use it like this:

$ karton logs --filter "karton.classifier"
[2021-03-29 18:06:50,063][INFO] Logger karton.cli-logger started
INFO:karton.cli-logger:Logger karton.cli-logger started
[INFO] karton.classifier: Received new task - 9d1367e3-24c3-4d29-b9cc-0af5b40c9b87
[INFO] karton.classifier: Classified b'qenwrvo6.tar' as {'type': 'sample', 'stage': 'recognized', 'quality': 'low', 'kind': 'runnable', 'platform': 'win32', 'extension': 'dll'} and tag runnable:win32:dll
[INFO] karton.classifier: Task done - 9d1367e3-24c3-4d29-b9cc-0af5b40c9b87
[INFO] karton.classifier: Received new task - b61ebbf3-e6bd-4e61-8360-00e9f71e90be
[INFO] karton.classifier: Classified b'9c414e070678797a964ebf5aea3dc8093bd41999' as {'type': 'sample', 'stage': 'recognized', 'quality': 'low', 'kind': 'runnable', 'platform': 'win32', 'extension': 'dll'} and tag runnable:win32:dll
[INFO] karton.classifier: Task done - b61ebbf3-e6bd-4e61-8360-00e9f71e90be

feature: More metrics

It's not a well-known fact, but we export prometheus-compatible metrics from the Karton dashboard. Recently we've exposed Karton metrics to the backend interface (#109), and used them in the dashboard. You can use them in Grafana to get graphs of tasks per second like this:

(We're going to document all of this sooner or later, stay tuned)

improvement: LocalResource content lazy-load

When LocalResource is constructed from path, user can use .content property to lazy-load content when necessary (#110). This used to return None.

improvement: Minor fixes in typing and documentation