* Removing all pragma from eigen.h

* Removing -Werror or equivalent from tests/CMakeLists.txt

* Restoring tests/CMakeLists.txt from master.

* Adding 4 PYBIND11_SILENCE_MSVC_C4127.

* Compatibility with -Wconversion, -Wdeprecated

* Introducing PYBIND11_COMPATIBILITY_WDEPRECATED_COPY

* Systematically using --verbose for compilations where possible (cmake 3.14 or newer).

Also changing all `cmake -t` to `--target`, `-v` to `--verbose`, `check` to `pytest`, for consistency (to make it easier to pin-point all commands of a certain type).

Also removing one `-j 2` for `pytest` in hopes of reducing flakes due to races in test_iostream and in prints from destructors.

* Commenting out pragmas as an experiment to reproduce previous observation.

* Removing all (newly added, but already commented-out) pragma code, adding HINT use -isystem (as cmake does).

* Restoring ci.yml from master. Those changes are better handled separately. BTW: in the last CI run there was still a test_iostream flake, even without the -j 2 for running the tests (verfied by inspecting the log).