Fix inclusion of py.typed in pip packages (#168)

The keys in the `package_data` dictionary in `setup.py` need to be
package names, not source directories, for the data files to be packaged
correctly.

setup.py

@@ -18,7 +18,7 @@
     author="Zakaria Zajac",
     author_email="zak@madzak.com",
     package_dir={'': 'src'},
-    package_data={"src/pythonjsonlogger": ["py.typed"]},
+    package_data={"pythonjsonlogger": ["py.typed"]},
     packages=find_packages("src", exclude="tests"),
     # https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires
     python_requires=">=3.6",