Fixing pybind11::bytes() ambiguous conversion issue.

[rwgk]

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
#2340, #2348, and #2409; this is best ignored.

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.

[YannickJadoul]

These changes were already once approved and merged (i.e., #2340), but not into master, since more refactoring is/was happening on str/bytes. (Later on, #2390 drastically improved the first proposed changes.)

For completeness: confusingly, the non-test code changes travelled through PR
#2340, #2348, and #2409; this is best ignored.

For historic and traceability purposes, #2340 is important and should not be ignored. The missing overload was first discovered in #2340 (comment), and I discovered and proposed the current solution later in this thread: #2340 (comment). Further down are links and the explanation on where this issue originated in pybind11's history (and why the current situation is most likely unintended).

Also, #2348 shows these changes in relation to the other proposed changes to the py::str and py::bytes types.

[YannickJadoul]

That being said, I do think we figured out by now that these changes are independent enough of the other pending changes to str and bytes, so this can probably be merged independently.

[rwgk]

Thank you @YannickJadoul and @henryiii !