Python 3.6.2 doesn't work with dbt 0.17.0
### Describe the bug
Running dbt on python <= 3.6.2 results in an error that `name 'TimestampSnapshotConfig' is not defined`. 3.6.3 is unaffected.

### Steps To Reproduce
Install python 3.6.2
Install dbt
Try to use dbt

### Expected behavior
dbt should run, not crash, etc


### System information
**Which database are you using dbt with?**
Any

**The output of `dbt --version`:**
```
0.17.0
```

**The operating system you're using:**
macos, linux

**The output of `python --version`:**
`Python 3.6.2`


diff --git a/core/setup.py b/core/setup.py
index b63daad5d..82fc31ae8 100644
--- a/core/setup.py
+++ b/core/setup.py
@@ -86,5 +86,5 @@ setup(
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
     ],
-    python_requires=">=3.6.2",
+    python_requires=">=3.6.3",
 )
['core/setup.py']