Update issue templates

Author: cjw296

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,30 @@
+---
+name: Bug report
+about: Only report bugs here that are specific to this backport.
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+This package is a rolling backport of [`unittest.mock`](https://github.com/python/cpython/blob/master/Lib/unittest/mock.py). 
+As such, any problems you encounter most likely need to be fixed upstream.
+
+Before submitting an issue here, please try and reproduce the problem on the latest release of Python 3, including alphas, and replace any import from `mock` with ones from `unittest.mock`.
+
+If the issue still occurs, then please report upstream through https://bugs.python.org/ as it will need to be fixed there so that it can be backported here and released to you.
+
+If the issue does not occur upstream, please file an issue using the template below as it may be an issue specific to the backport:
+
+**What versions are you using?**
+ - Python: [e.g. 3.7.1]
+ - Mock: [e.g. 4.0.2]
+ - Operating System: [e.g.Linux, macOS, Windows]
+
+**What happened?**
+
+<!-- 
+A clear and concise description of what the problem is, including full tracebacks and code being executed  
+-->
+
+**What were you hoping to happen instead?**