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* `#error BYE_BYE_GOLDEN_SNAKE`

* Removing everything related to 2.7 from ci.yml

* Commenting-out Centos7

* Removing `PYTHON: 27` from .appveyor.yml

* "PY2" removal, mainly from tests. C++ code is not touched.

* Systematic removal of `u` prefix from `u"..."` and `u'...'` literals. Collateral cleanup of a couple minor other things.

* Cleaning up around case-insensitive hits for `[^a-z]py.*2` in tests/.

* Removing obsolete Python 2 mention in compiling.rst

* Proper `#error` for Python 2.

* Using PY_VERSION_HEX to guard `#error "PYTHON 2 IS NO LONGER SUPPORTED.`

* chore: bump pre-commit

* style: run pre-commit for pyupgrade 3+

* tests: use sys.version_info, not PY

* chore: more Python 2 removal

* Uncommenting Centos7 block (PR #3691 showed that it is working again).

* Update pre-commit hooks

* Fix pre-commit hook

* refactor: remove Python 2 from CMake

* refactor: remove Python 2 from setup code

* refactor: simplify, better static typing

* feat: fail with nice messages

* refactor: drop Python 2 C++ code

* docs: cleanup for Python 3

* revert: intree

revert: intree

* docs: minor touchup to py2 statement
Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>
Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>

* Call PySys_SetArgv when initializing interpreter.

* Document argc/argv parameters in initialize_interpreter.

* Remove manual memory management from set_interpreter_argv in favor of smart pointers.

* Use size_t for indexers in set_interpreter_argv.

* Minimize macros for flow control in set_interpreter_argv.

* Fix 'unused variable' warning on Py2

* whitespace

* Define wide_char_arg_deleter outside set_interpreter_argv.

* Do sys.path workaround in C++ rather than eval.

* Factor out wchar conversion to a separate function.

* Restore widened_argv variable declaration.

* Fix undeclared widened_arg variable on some paths.

* Use delete[] to match new wchar_t[].

* Fix compiler errors

* Use PY_VERSION_HEX for a cleaner CVE-2008-5983 mode check.

* Fix typo

* Use explicit type for deleter so delete[] works cross-compiler.

* Always use PySys_SetArgvEx because pybind11 doesn't support pythons that don't include it.

* Remove pointless ternary operator.

* Use unique_ptr.reset instead of a second initialization.

* Rename add_program_dir_to_path parameter to clarify intent.

* Add defined() check before evaluating HAVE_BROKEN_MBSTOWCS.

* Apply clang-tidy fixes

* Pre-commit

* refactor: use const for set_interpreter_argv

* Try to fix const issue and allocate vector properly

* fix: copy strings on Python 2

* Applying clang-format-diff relative to master.

The only manual change is an added empty line between pybind11 and system `#include`s.

```
git diff -U0 --no-color master | python3 $HOME/clone/llvm-project/clang/tools/clang-format/clang-format-diff.py -p1 -style=file -i
```
Co-authored-by: Boris Staletic <boris.staletic@gmail.com>
Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>
Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>
Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>

At this point, there is only a single test for interpreter basics.

Apart from embedding itself, having a C++ test framework will also
benefit the C++-side features by allowing them to be tested directly.