Get rid of BLOCKED label in favor of [WIP] subject prefix (#1956)

We (I) introduced the BLOCKED label ~2 years ago, but it never got much usage, and I remember it startled some developers (Ivan may recall...). A better convention seems to put [WIP] in the subject, so let's standardize on that and get rid of the BLOCKED label.

I already removed the BLOCKED label and edited the subject of the one issue under it.

We also don't make good use of the other labels, but they seem more useful and less controversial.

CONTRIBUTING.md

@@ -313,8 +313,6 @@ missing, incomplete or incorrect) and we won't add noise to the
 tracker by labeling all of them.  Here's what our labels mean.  (We
 also apply these to pull requests.)
 
-* **blocked**: This issue is waiting for the resolution of some issue
-    external to typeshed.
 * **bug**: It's a bug in a stub.
 * **bytes-unicode**: It's related to bytes vs. unicode, usually Python 2.
 * **feature**: It's a new typeshed feature.
@@ -326,6 +324,11 @@ also apply these to pull requests.)
 * **size-medium**: An issue of average complexity.
 * **size-small**: An issue that will take only little effort to fix.
 
+Sometimes a PR can't make progress until some external issue is
+addressed.  We indicate this by editing the subject to add a ``[WIP]``
+prefix.  (This should be removed before committing the issue once
+unblocked!)
+
 ### Core developer guidelines
 
 Core developers should follow these rules when processing pull requests: