No dep on slack-sdk

[wRAR]

In #338 slackclient was replaced by slack-sdk in setup.py only in test_requirements but not in extras_require, so installing spidermon[monitoring] doesn't install it and the Slack action no longer works.

[rennerocha]

As a fallback, installing slack-sdk manually using pip install slack-sdk and/or adding slack-sdk in the requirements would be enough.

Perhaps monitoring section should be updated/removed in future. Using pip install spidermon[slack] (when we need Slack), pip install spidermon[amazon] (when using Amazon stuff) or pip install spidermon[jsonschema] when using JSON Schema makes more sense to me and avoid us to install libraries that won't be used and keep the libraries installed more explicit.