fix: remove redundant copy operation to fix warning (#3486)

* fix compiler warning: deprecated implicit copy constructor

* take care of the bug http://eigen.tuxfamily.org/bz/show_bug.cgi?id=747

* add parenthesis for better reading

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci



* Update include/pybind11/eigen.h

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>