gnu: python-pytest: Update to 5.2.1.

[Tanguy Le Carrour <tanguy@bioneland.org>]

Hi Guix!

I'm working on updating python-pytest and… not everything is going as
smoothly as I was expecting! :-(

Pytest 5.2.1 depends on Pluggy >=0.12.0… which depends on
importlib-metadata. But when I had the latter as a propagated-input to
python-pluggy, then `guix build` gets stuck.

    diff --git a/gnu/packages/python-xyz.scm b/gnu/packages/python-xyz.scm
    index a7f83dcd31..ab80cdebf9 100644
    --- a/gnu/packages/python-xyz.scm
    +++ b/gnu/packages/python-xyz.scm
    @@ -8914,17 +8914,19 @@ library as well as on the command line.")
     (define-public python-pluggy
       (package
        (name "python-pluggy")
    -   (version "0.11.0")
    +   (version "0.13.0")
        (source
         (origin
          (method url-fetch)
          (uri (pypi-uri "pluggy" version))
          (sha256
           (base32
    -       "10511a54dvafw1jrk75mrhml53c7b7w4yaw7241696lc2hfvr895"))))
    +       "0d4gsvb4kjqhiqqi4bbsdp7s1xlyl5phibcw1q1mrpd65xia2pzs"))))
        (build-system python-build-system)
        (native-inputs
         `(("python-setuptools-scm" ,python-setuptools-scm)))
    +   (propagated-inputs
    +    `(("python-importlib-metadata" ,python-importlib-metadata)))
        (synopsis "Plugin and hook calling mechanism for Python")
        (description "Pluggy is an extraction of the plugin manager as used by
     Pytest but stripped of Pytest specific details.")

The problem seems to be that python-importlib-metadata's dependency graph
contains… python-pluggy!

How am I supposed to fix this chicken and egg situation?! Is this
something that should be notified to upstream?!

Regards

-- 
Tanguy