fix: correct open encoding

[TendTo]

Using "utf8" instead of "utf-8" to indicate the encoding seems to create issues on Windows, making it unable to install the package.

Now, the python documentation of open uses says that you can use any encoding from here, which states that 'utf-8' is a valid alias for the 'utf_8' codec.
To be fair, "utf8" should work too, but I assume this is not properly supported on Windows.

In any case, all the official examples I could find from the python documentation use "utf-8" (example), so that is probably the safest option.

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional information

Tested on

• Windows 10
• Python 3.10 (conda environment)