Symbol not found find_last_not_of
I believe I've encountered the issue reported in #257.

Using Python 3.9.2 as installed from Python.org on MacOS 11.2.3, I've used `brew install sdl2 sdl2_image gifsicle` and then install pyxel 1.4.3 from wheel, then attempt to install the examples, but it fails with this error:

```
draft $ install_pyxel_examples
Traceback (most recent call last):
  File "/Users/jaraco/.local/bin/install_pyxel_examples", line 5, in <module>
    from pyxel.examples import install
  File "/Users/jaraco/.local/pipx/venvs/pyxel/lib/python3.9/site-packages/pyxel/__init__.py", line 10, in <module>
    from . import core  # type: ignore
  File "/Users/jaraco/.local/pipx/venvs/pyxel/lib/python3.9/site-packages/pyxel/core/__init__.py", line 42, in <module>
    _lib = _load_library()
  File "/Users/jaraco/.local/pipx/venvs/pyxel/lib/python3.9/site-packages/pyxel/core/__init__.py", line 39, in _load_library
    return cdll.LoadLibrary(lib_path)
  File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/ctypes/__init__.py", line 452, in LoadLibrary
    return self._dlltype(name)
  File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/ctypes/__init__.py", line 374, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: dlopen(/Users/jaraco/.local/pipx/venvs/pyxel/lib/python3.9/site-packages/pyxel/core/bin/macos/libpyxelcore.dylib, 6): Symbol not found: __ZNKSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEE16find_last_not_ofEPKcmm
  Referenced from: /Users/jaraco/.local/pipx/venvs/pyxel/lib/python3.9/site-packages/pyxel/core/bin/macos/libpyxelcore.dylib
  Expected in: /usr/lib/libstdc++.6.dylib
 in /Users/jaraco/.local/pipx/venvs/pyxel/lib/python3.9/site-packages/pyxel/core/bin/macos/libpyxelcore.dylib
```