Create rule S6899 (#3589)

rules/S6899/metadata.json

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+{
+}

rules/S6899/python/metadata.json

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+{
+  "title": "datetime.datetime objects should not be compared with datetime.date objects",
+  "type": "BUG",
+  "status": "ready",
+  "remediation": {
+    "func": "Constant\/Issue",
+    "constantCost": "5min"
+  },
+  "tags": [
+  ],
+  "defaultSeverity": "Major",
+  "ruleSpecification": "RSPEC-6899",
+  "sqKey": "S6899",
+  "scope": "All",
+  "defaultQualityProfiles": ["Sonar way"],
+  "quickfix": "unknown",
+  "code": {
+    "impacts": {
+      "RELIABILITY": "HIGH"
+    },
+    "attribute": "LOGICAL"
+  }
+}

rules/S6899/python/rule.adoc

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+This rule raises an issue when trying to perform comparison or arithmetic operations between `datetime.date` and `datetime.datetime` objects.
+
+== Why is this an issue?
+Despite the fact that the `datetime.datetime` is inherited from the `datetime.date` they are not compatible from comparison and arithmetical operations perspective.
+Trying to compare a `datetime.datetime` with a `datetime.date` objects leads to a `TypeError`.
+
+== How to fix it
+Make sure that comparison or arithmetic operations are performed between compatible types e.g. convert `datetime.datetime` to `datetime.date` or vice versa.
+
+=== Code examples
+
+==== Noncompliant code example
+
+[source,python,diff-id=1,diff-type=noncompliant]
+----
+from datetime import datetime, date
+
+dt = datetime.now()
+d = date.today()
+
+if dt < d: # Noncompliant: TypeError: can't compare datetime.datetime to datetime.date
+    ...
+----
+
+==== Compliant solution
+
+[source,python,diff-id=1,diff-type=compliant]
+----
+from datetime import datetime, date
+
+dt = datetime.now()
+d = date.today()
+
+if dt.date() < d: # OK
+    ...
+----
+
+//=== How does this work?
+
+//=== Pitfalls
+
+//=== Going the extra mile
+
+
+== Resources
+=== Documentation
+* Python documentation - https://docs.python.org/3/library/datetime.html#datetime-objects[Datetime Objects]
+* Python documentation - https://docs.python.org/3/library/datetime.html#date-objects[Date Objects]
+//=== Articles & blog posts
+//=== Conference presentations
+//=== Standards
+//=== External coding guidelines
+//=== Benchmarks