Vendor typing_extensions

We are using more and more typing features in Spack, and without features like
protocols, typing core is becoming harder and harder.

I think it's worth vendoring `typing_extensions` for this. It will get us a number of
useful capabilities:

* `Literal`
* `TypedDict`
* `Protocol`

among others.

Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>

COPYRIGHT

@@ -102,6 +102,6 @@ PackageName: sbang
 PackageHomePage: https://github.com/spack/sbang
 PackageLicenseDeclared: Apache-2.0 OR MIT
 
-PackageName: six
-PackageHomePage: https://pypi.python.org/pypi/six
-PackageLicenseDeclared: MIT
+PackageName: typing_extensions
+PackageHomePage: https://pypi.org/project/typing-extensions/
+PackageLicenseDeclared: Python-2.0