Adapt friendclass to Doxygen 1.9

[jakobandersen]

From Doxygen 1.9.0 the XML for friend class declarations has changed, e.g.,::

       <memberdef kind="friend" id="classBar_1a549e901751cbd1b0dd39326270e23ca6" prot="private" static="no" const="no" explicit="no" inline="no" virt="non-virtual">
-        <type>friend class</type>
+        <type>class</type>
         <definition>friend class Foo</definition>
         <argsstring></argsstring>
         <name>Foo</name>
+        <param>
+          <type><ref refid="classFoo" kindref="compound">Foo</ref></type>
+        </param>
         <briefdescription>
         </briefdescription>
         <detaileddescription>
         </detaileddescription>
         <inbodydescription>
         </inbodydescription>
         <location file="foo.h" line="6" column="5" bodyfile="foo.h" bodystart="6" bodyend="-1"/>
       </memberdef>

Fixes #616.

[jakobandersen]

I think the CI will go further when #617 has been merged.

[vermeeren]

Good catch @jakobandersen , indeed makes sense based upon XML diff. I'll check if the tests pass properly after #617 is merged for Sphinx 3.4.x compatibility.