Fix find_packages() to not include tests packages

The list of packages we install with find_packages() is now:

```
>>> pprint(find_packages(exclude=['tests*']))
['botocore',
 'botocore.vendored',
 'botocore.vendored.requests',
 'botocore.vendored.requests.packages',
 'botocore.vendored.requests.packages.charade',
 'botocore.vendored.requests.packages.urllib3',
 'botocore.vendored.requests.packages.urllib3.contrib',
 'botocore.vendored.requests.packages.urllib3.packages',
 'botocore.vendored.requests.packages.urllib3.packages.ssl_match_hostname']
```

Also manually verified we don't install `tests` with this change.