fix: Use include rather than exclude to find_namespace_packages in setup.py (#502)

https://setuptools.pypa.io/en/latest/userguide/package_discovery.html
has a warning:

> Please have in mind that find_namespace: (setup.cfg),
> find_namespace_packages() (setup.py) and find (pyproject.toml)
> will scan all folders that you have in your project directory
> if you use a flat-layout.

That applies here.  Without this change, if you run:
'python3 setup.py bdist_wheel' then you may end up with
build/ in your wheel, and in some cases even
docs/ and testing/ .

The fix is to only include proto and proto.*.

Signed-off-by: Scott Moser <smoser@brickies.net>
Co-authored-by: Anthonios Partheniou <partheniou@google.com>

Author: smoser

setup.py

@@ -37,7 +37,7 @@
     author="Google LLC",
     author_email="googleapis-packages@google.com",
     url="https://github.com/googleapis/proto-plus-python.git",
-    packages=find_namespace_packages(exclude=["docs", "tests"]),
+    packages=find_namespace_packages(include=["proto", "proto.*"]),
     description="Beautiful, Pythonic protocol buffers.",
     long_description=README,
     platforms="Posix; MacOS X",