The only and rather tedious way I can think of is extract the xlsx-file (it is only a zip archive) and modify the sharedStrings.xml in there (e.g. using ElementTree). 
Afterwards zip the file back together.
The approach is described here: 
https://stackoverflow.com/a/53454150/7919597
E.g. for a cell like 

the sharedStrings.xml looks like
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
  <sst xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" count="1" uniqueCount="1">
    <si>
      <r>
        <rPr>
          <sz val="11"/><color rgb="FFFF0000"/><rFont val="Calibri"/><family val="2"/>
          <scheme val="minor"/>
         </rPr>
         <t xml:space="preserve">Some Text</t>
      </r>
      <r>
        <rPr>
           <sz val="11"/><color theme="7" tint="-0.249977111117893"/><rFont val="Calibri"/><family val="2"/><scheme val="minor"/>
         </rPr>
       <t>Another Text</t>
    </r>
  </si>
</sst>