Source and editable installs fail in some locales

Methods of installing GitPython that run code from `setup.py` fail in some locales. This does not affect installing from a wheel, but it does affect installing from an sdist, or installing from a local directory, including the editable install procedure recommended for development in the readme. The same problem happens when building GitPython. Building or installing using the old method of running `setup.py` directly is also affected. The error is of this form, though the codec will not always be `gbk`:



```text

UnicodeDecodeError: 'gbk' codec can't decode byte 0xa6 in position 473: illegal multibyte sequence

```



I am unsure if this ever happens in practice on Unix-like systems, whose locales are usually UTF-8. However, it happens on Windows systems in which `README.md` cannot be decoded using the system's active ANSI code page. This is a rarely-changed systemwide setting, so changing the user preferred languages, display language, or input method are not workarounds. I discovered this on a Simplified Chinese (zh-CN) build of Windows Server 2022 while using it to test some WSL-related test helper logic in #1745. Such a system uses ANSI code page 936. `README.md` is UTF-8, but it currently happens that it can be decoded with code page 1252, which Windows builds for Western European languages use as their ANSI code page. I expect encodings other than cp936 to fail as well.



With the PyPI sdist for GitPython 3.1.40 (on Python 3.12 x86-64, though I expect all supported versions to be affected):



```text

(.venv) C:\Users\Administrator\gptest> pip install --no-binary GitPython GitPython

Collecting GitPython

  Using cached GitPython-3.1.40.tar.gz (200 kB)

  Installing build dependencies ... done

  Getting requirements to build wheel ... error

  error: subprocess-exited-with-error



  × Getting requirements to build wheel did not run successfully.

  │ exit code: 1

  ╰─> [18 lines of output]

      Traceback (most recent call last):

        File "C:\Users\Administrator\gptest\.venv\Lib\site-packages\pip\_vendor\pyproject_hooks\_in_process\_in_process.py", line 353, in <module>

          main()

        File "C:\Users\Administrator\gptest\.venv\Lib\site-packages\pip\_vendor\pyproject_hooks\_in_process\_in_process.py", line 335, in main

          json_out['return_val'] = hook(**hook_input['kwargs'])

                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

        File "C:\Users\Administrator\gptest\.venv\Lib\site-packages\pip\_vendor\pyproject_hooks\_in_process\_in_process.py", line 118, in get_requires_for_build_wheel

          return hook(config_settings)

                 ^^^^^^^^^^^^^^^^^^^^^

        File "C:\Users\Administrator\AppData\Local\Temp\pip-build-env-8j1uxmuv\overlay\Lib\site-packages\setuptools\build_meta.py", line 325, in get_requires_for_build_wheel

          return self._get_build_requires(config_settings, requirements=['wheel'])

                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

        File "C:\Users\Administrator\AppData\Local\Temp\pip-build-env-8j1uxmuv\overlay\Lib\site-packages\setuptools\build_meta.py", line 295, in _get_build_requires

          self.run_setup()

        File "C:\Users\Administrator\AppData\Local\Temp\pip-build-env-8j1uxmuv\overlay\Lib\site-packages\setuptools\build_meta.py", line 311, in run_setup

          exec(code, locals())

        File "<string>", line 20, in <module>

      UnicodeDecodeError: 'gbk' codec can't decode byte 0xa6 in position 473: illegal multibyte sequence

      [end of output]



  note: This error originates from a subprocess, and is likely not a problem with pip.

error: subprocess-exited-with-error



× Getting requirements to build wheel did not run successfully.

│ exit code: 1

╰─> See above for output.



note: This error originates from a subprocess, and is likely not a problem with pip.

```



A workaround is to pass `-X utf8` to `python`. For example, `python -X utf8 -m pip install ...` can be used for installation.



The fix should be straightforward. Importing `setup.py` confirms that the specific cause is reading `README.md`:



```text

(.venv) C:\Users\Administrator\repos\GitPython [main ≡]> python -c 'import setup'

Traceback (most recent call last):

  File "<string>", line 1, in <module>

  File "C:\Users\Administrator\repos\GitPython\setup.py", line 20, in <module>

    long_description = rm_file.read()

                       ^^^^^^^^^^^^^^

UnicodeDecodeError: 'gbk' codec can't decode byte 0xa6 in position 477: illegal multibyte sequence

```



It can be fixed by passing `encoding="utf-8"`. **I have proposed this change in #1748.**



gitdb and smmap are unaffected, because [gitdb does not open files to read those data in `setup.py`](https://github.com/gitpython-developers/gitdb/blob/ec58b7e55c8fec2479290a22faa293799f6805fc/setup.py#L24), while [smmap does open `README.md` but passes `encoding="utf-8"`](https://github.com/gitpython-developers/smmap/blob/f1ace75be355fdec927793e462b9b12bf6ec9520/setup.py#L14).

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