Create a pull request template

Author: brettcannon

.github/pull_request_template.md

@@ -0,0 +1,8 @@
+Fixes #
+
+This pull request:
+- [ ] Has a title summarizes what is changing
+- [ ] Includes a [news entry](https://github.com/Microsoft/vscode-python/tree/master/news) file
+- [ ] Has unit tests & [code coverage](https://codecov.io/gh/Microsoft/vscode-python) is not adversely affected (within reason)
+- [ ] Works on all [actively maintained versions of Python](https://devguide.python.org/#status-of-python-branches) (e.g. Python 2.7 & the latest Python 3 release)
+- [ ] Works on Windows 10, macOS, and Linux (e.g. considered file system case-sensitivity)