gnu: python-docutils: Update to 0.21.2.

This change updates python-docutils to the latests version and pins some
older packages on 0.19 to resolve build documentation issue which uses
rst2man.

* gnu/packages/python-xyz.scm (python-docutils): Update to 0.21.2.
[build-system]: Use pyproject.
[arguments] <phases>: Do not overwrite 'check use {#:test-backend} and
{#:test-flags}.
[native-inputs]: Add python-flit-core.

(python-docutils-0.16)[native-inputs]: Add python-wheel.

(python-docutils-0.19): New variable.

* gnu/packages/diffoscope.scm (trydiffoscope) [native-inputs]: Remove
python-docutils; add python-docutils-0.19.
* gnu/packages/gtk.scm (gtk): Likewise.
* gnu/packages/linux.scm (rdma-core): Likewise.
* gnu/packages/power.scm (apcupsd): Likewise.
* gnu/packages/version-control.scm (cgit): Likewise.
* gnu/packages/virtualization.scm (ganeti)) Likewise.

Change-Id: If58b9b12732347245e66e3c2cb5e67b0a8450913

gnu/packages/gtk.scm

@@ -1373,7 +1373,7 @@ application suites.")
            pkg-config
            python-pygobject
            ;; These python modules are required for building documentation.
-           python-docutils
+           python-docutils-0.19
            python-jinja2
            python-markdown
            python-markupsafe