Add py.typed marker and include it in package data

Added py.typed file to root so that typing tools such as Mypy recognize
dogpile as typed. Pull request courtesy Daverball.

Fixes: #238
Closes: #239
Pull-request: #239
Pull-request-sha: 345e6cf

Change-Id: I1006601759b2369a8090d1b6117145c6e02e986c

Author: Daverball

docs/build/unreleased/238.rst

@@ -0,0 +1,6 @@
+.. change::
+    :tags: bug, typing
+    :tickets: 238
+
+    Added py.typed file to root so that typing tools such as Mypy recognize
+    dogpile as typed. Pull request courtesy Daverball.

dogpile/py.typed

@@ -27,6 +27,7 @@ install_requires =
 zip_safe = False
 packages = find:
 python_requires = >=3.6
+include_package_data = True
 package_dir =
     =.
 
@@ -38,6 +39,9 @@ exclude =
 [options.exclude_package_data]
 '' = tests*
 
+[options.package_data]
+* = py.typed
+
 [options.entry_points]
 mako.cache =
     dogpile.cache = dogpile.cache.plugins.mako_cache:MakoPlugin