Move Rule into a dataclass (#1029)

* WIP: Convert Rule to a dataclass
* Fix make release
* Lint fixes
* Remove dead code
* Fix lint and tests
* Use Python 3.8 in GitHub actions
* Update README to 3.8+
* Add Python 3.8 assertion
* Fix is_dirty property
* Remove incorrect pop from contents
* Add mixin with from_dict() and to_dict() methods
* Bypass validation for deprecated rules
* Fix rule_prompt
* Fix dict_hash usage
* Fix rule_event_search
* Switch to definitions.Date
* Fix toml-lint command, ignoring 'unneeded defaults'
* Moved severity Literal to definitions.Severity
* Remove BaseMarshmallowDataclass
* Fix lint and tests
* Add maturity to metadata for rule prompt loop
* Fix typo in devtools
* Use rule loader to load single rule in toml-lint
* Add Schema hint to __schema method
* Add MITREAttackURL definition
* Fix is_dirty to compare sha<-->sha
* Normalize the autoformatted rule output for API and toml-lint
* Make the package hash match
* Make the rule object mutable but not rule contents
* Restore the rules

.github/workflows/pythonpackage.yml

@@ -14,10 +14,10 @@ jobs:
     steps:
     - uses: actions/checkout@v2
 
-    - name: Set up Python 3.7
+    - name: Set up Python 3.8
       uses: actions/setup-python@v2
       with:
-        python-version: 3.7
+        python-version: 3.8
 
     - name: Install dependencies
       run: |