Refactor python algorithm import (#5657)

* Python research import improvements

- Improve start.py for research env
- Remove unrequired imports

* Centralize algorithm imports

* Add regression test GH action

* Unit test python import clean up

* Join research and main imports

* More python import clean up

* Fix failing skipped regression algorithm

Algorithm.Python/DropboxCoarseFineAlgorithm.py

@@ -11,17 +11,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from clr import AddReference
-AddReference("System")
-AddReference("QuantConnect.Algorithm")
-AddReference("QuantConnect.Common")
-
-from System import *
-from QuantConnect import *
-from QuantConnect.Data import *
-from QuantConnect.Algorithm import *
-from QuantConnect.Securities import *
-from datetime import datetime
+from AlgorithmImports import *
 
 ### <summary>
 ### In this algortihm, we fetch a list of tickers with corresponding dates from a file on Dropbox.