Added requirement for xmltodict to spec

Actually the real issue to this defect seems to live in the
anymarkup source code. It seems that the import of xmltodict
at the beginning of the core module is optional, but later on
when handling XML xmltodict seems to be mandatory, see here:
https://github.com/bkabrda/anymarkup-core/blob/08896a8215819edcc413e3f2588518046
9a4c2ed/anymarkup_core/__init__.py#L210-L229. Because this
issue can result in a "ImportError: Can't parse xml: xmltodict not installed"
turning kiwi into a bad exit condition I propose to add
this python requirement to the kiwi spec as a workaround and
try fixing the anymarkup packaging and module setup upstream
in addition. To my knowledge all relevant distributions
provides xmltodict

package/python-kiwi-spec-template

@@ -371,6 +371,7 @@ Requires:       python%{python3_pkgversion}-docopt
 Requires:       python%{python3_pkgversion}-lxml
 Requires:       python%{python3_pkgversion}-requests
 Requires:       python%{python3_pkgversion}-setuptools
+Requires:       python%{python3_pkgversion}-xmltodict
 %if 0%{?rhel} || 0%{?fedora}
 Requires:       (python%{python3_pkgversion}-typing-extensions if python%{python3_pkgversion} < 3.8)
 %endif