Add docstrings and typing to db_engine_specs and sql_parse (apache#8058)

* Add typing to db_engine_specs

* Add more type annotations and docstrings

* Add docstrings and typing to sql_parse and db_engine_specs

* Refine select_star

* Fix execute and add more docstrings

* Revert kwargs change from execute

* Remove redundant or

* Align view and table getter schema types

* Fix return type of latest_partition

* Remove some typing from presto

* Improve docstring for __extract_from_token

superset/db_engine_specs/athena.py

@@ -15,6 +15,8 @@
 # specific language governing permissions and limitations
 # under the License.
 # pylint: disable=C,R,W
+from datetime import datetime
+
 from superset.db_engine_specs.base import BaseEngineSpec
 
 
@@ -38,7 +40,7 @@ class AthenaEngineSpec(BaseEngineSpec):
     }
 
     @classmethod
-    def convert_dttm(cls, target_type, dttm):
+    def convert_dttm(cls, target_type: str, dttm: datetime) -> str:
         tt = target_type.upper()
         if tt == "DATE":
             return "from_iso8601_date('{}')".format(dttm.isoformat()[:10])
@@ -47,14 +49,15 @@ def convert_dttm(cls, target_type, dttm):
         return "CAST ('{}' AS TIMESTAMP)".format(dttm.strftime("%Y-%m-%d %H:%M:%S"))
 
     @classmethod
-    def epoch_to_dttm(cls):
+    def epoch_to_dttm(cls) -> str:
         return "from_unixtime({col})"
 
     @staticmethod
-    def mutate_label(label):
+    def _mutate_label(label: str) -> str:
         """
         Athena only supports lowercase column names and aliases.
-        :param str label: Original label which might include uppercase letters
-        :return: String that is supported by the database
+
+        :param label: Expected expression label
+        :return: Conditionally mutated label
         """
         return label.lower()