Add: Py-Plotly (#6628)

* Add: Py-Plotly

The requirements of py-plotlit are rather strict fixed to specific
versions, so I did not add an "upper versions" `:` to the constrains.
See
  https://github.com/plotly/plotly.py/blob/master/requirements.txt

* Py-Requests, Py-Six, Py-Pytz: Older Versions

Add two older versions of requests, pytz and six for plotly.

var/spack/repos/builtin/packages/py-six/package.py

@@ -35,6 +35,7 @@ class PySix(PythonPackage):
 
     version('1.10.0', '34eed507548117b2ab523ab14b2f8b55')
     version('1.9.0',  '476881ef4012262dfc8adc645ee786c4')
+    version('1.8.0',  '1626eb24cc889110c38f7e786ec69885')
 
     extends('python', ignore=r'bin/pytest')