Migrate pythonnet to .NET Core 3.0

[gsalaz98]

What does this implement/fix? Explain your changes.

Migrates and adds support for .NET Core 3.0 for pythonnet. This is required for python algorithms to run in LEAN .NET Core

Does this close any currently open issues?

No

Any other comments?

I believe this might break backwards compatibility with .NET Framework 4.5.2 -- which is what LEAN currently runs on. Will have to do some testing for that.

Checklist

Check all those that are applicable and complete.

• [ ] Make sure to include one or more tests for your change
• [ ] If an enhancement PR, please create docs and at best an example
• [ ] Add yourself to AUTHORS
• [ ] Updated the CHANGELOG

[Martin-Molinero]

Closed in favor of https://github.com/QuantConnect/pythonnet/pull/47