Move code to a src directory

This is gaining traction in the community as well as avoiding some
potential testing pitfalls, see
https://hynek.me/articles/testing-packaging/

Author: pgjones

MANIFEST.in

@@ -2,7 +2,7 @@ graft compliance
 graft example
 graft docs
 graft test
-graft wsproto
+graft src/wsproto
 prune docs/build
 prune compliance/autobahntestsuite-venv
 prune compliance/reports

setup.py

@@ -13,7 +13,7 @@
 
 # Get the version
 version_regex = r'__version__ = ["\']([^"\']*)["\']'
-with open(os.path.join(PROJECT_ROOT, 'wsproto/__init__.py')) as file_:
+with open(os.path.join(PROJECT_ROOT, 'src/wsproto/__init__.py')) as file_:
     text = file_.read()
     match = re.search(version_regex, text)
 
@@ -35,9 +35,9 @@
     author='Benno Rice',
     author_email='[email protected]',
     url='https://github.com/python-hyper/wsproto/',
-    packages=find_packages(exclude=['test']),
+    packages=find_packages("src"),
+    package_dir={"": "src"},
     package_data={'': ['LICENSE', 'README.rst']},
-    package_dir={'wsproto': 'wsproto'},
     python_requires=">=3.6.1",
     include_package_data=True,
     license='MIT',