add tasks for updating vendored libs

tasks/vendoring.py

@@ -0,0 +1,23 @@
+from __future__ import absolute_import, print_function
+import py
+import invoke
+
+VENDOR_TARGET = py.path.local("_pytest/vendored_packages")
+GOOD_FILES = 'README.md', '__init__.py'
+
+@invoke.task()
+def remove_libs(ctx):
+    print("removing vendored libs")
+    for path in VENDOR_TARGET.listdir():
+        if path.basename not in GOOD_FILES:
+            print(" ", path)
+            path.remove()
+
+@invoke.task(pre=[remove_libs])
+def update_libs(ctx):
+    print("installing libs")
+    ctx.run("pip install -t {target} pluggy".format(target=VENDOR_TARGET))
+    ctx.run("git add {target}".format(target=VENDOR_TARGET))
+    print("Please commit to finish the update after running the tests:")
+    print()
+    print('    git commit -am "Updated vendored libs"')