Update license information in pyproject.toml (#1776)

Update license information in pyproject.toml
- Fixes license specifiers according to https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license-and-license-files
- This allows automatic license inspection tools like pip-licenses to correctly categorize this package.

Author: ecederstrand

pyproject.toml

@@ -8,6 +8,7 @@ name="tableauserverclient"
 dynamic = ["version"]
 description='A Python module for working with the Tableau Server REST API.'
 authors = [{name="Tableau", email="github@tableau.com"}]
+license = "MIT"
 license-files = ["LICENSE"]
 readme = "README.md"