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
DOC: corrections in v0.14.0
BUG: multi-index output formatting
Merge pull request #7232 from bjonen/fix_7174

BUG: multi-index output formatting is buggy (GH7174)
make more nanops work with complex numbers
add tests for nanops
implement additional tests for groupby apply methods
simplify groupby's std method
add sem to nanops
add sem to aggregate methods
add sem to groupby
add sem to docs