release rc1.

We had not big complaint since beta1, so doing an RC.
Main issues where:

  -- passing HistManager.hist_file twice fails in 5.x not in 4.x
  -- easy_install does not respect python_requires.

traitlets/_version.py

@@ -1,4 +1,4 @@
-version_info = (5, 0, 0, "b1")
+version_info = (5, 0, 0, "rc1")
 
 # unlike `.dev`, alpha, beta and rc _must not_ have dots,
 # or the wheel and tgz won't look to pip like the same version.