Modify PythonSettings.getInstance to default to current workspace

[DonJayamanne]

If nothing is passed into getInstance method as the value for the resource arguement, then use the current workspace as the default.

• This prevents VS Code from printing warnings such as the following:

[ms-python.python] Accessing a resource scoped configuration without providing a resource is not expected. To get the effective value for 'python.xxx', provide the URI of a resource or 'null' for any resource.

[DonJayamanne]

Closing as this is how the extension works today.
The problem was in the Unit test (Configuration Settings: Check Values) which has been resolve since.