TST/DOC/BUG: Accommodate OpenPyXL incompatibility

*   Detect whether a compatible version is installed
*   Skip openpyxl unit tests instead of failing
*   Inhibit registration of `openpyxl` engine in `ExcelWriter`
*   Raise `UserWarning`
*   Document limitation as a known issue
*   Resolve issue #7169
Merge pull request #7214 from neirbowj/openpyxl_compat

TST/DOC: Test for and document incompatibility with openpyxl 2.0.0 and later