Adding missing `bytes` type to `test_constructors()`, to exercise the code change.

The changes in the PR were cherry-picked from PR #2409 (with a very minor
modification in test_pytypes.py related to flake8). Via PR #2409, these
changes were extensively tested in the Google environment, as summarized here:
https://docs.google.com/document/d/1TPL-J__mph_yHa1quDvsO12E_F5OZnvBaZlW9IIrz8M/
The changes in this PR did not cause an issues at all.

Note that `test_constructors()` before this PR passes for Python 2 only
because `pybind11::str` can hold `PyUnicodeObject` or `PyBytesObject`. As a
side-effect of this PR, `test_constructors()` no longer relies on this
permissive `pybind11::str` behavior. However, the permissive behavior is still
exercised/exposed via the existing `test_pybind11_str_raw_str()`.

The test code change is designed to enable easy removal later, when Python 2
support is dropped.

For completeness: confusingly, the non-test code changes travelled through PR

Example `ambiguous conversion` error fixed by this PR:
```
pybind11/tests/test_pytypes.cpp:214:23: error: ambiguous conversion for functional-style cast from 'pybind11::detail::item_accessor' (aka 'accessor<accessor_policies::generic_item>') to 'py::bytes'
            "bytes"_a=py::bytes(d["bytes"]),
                      ^~~~~~~~~~~~~~~~~~~~
pybind11/include/pybind11/detail/../pytypes.h:957:21: note: candidate constructor
    PYBIND11_OBJECT(bytes, object, PYBIND11_BYTES_CHECK)
                    ^
pybind11/include/pybind11/detail/../pytypes.h:957:21: note: candidate constructor
pybind11/include/pybind11/detail/../pytypes.h:987:15: note: candidate constructor
inline bytes::bytes(const pybind11::str &s) {
              ^
1 error generated.
```