Fix/utf8 encoding

[IndigoCarmine]

Description

I added the encoding="utf-8" argument so that model source files are explicitly read as UTF-8.

This fixes the UnicodeDecodeError that occurs when building plugin models on Windows systems using a non-UTF-8 locale, such as the Japanese locale.
Fixes #4095

How Has This Been Tested?

Tested the change on Windows with a Japanese locale by building the plugin models.
The build completed successfully without the UnicodeDecodeError using SasView/sasmodels#754.

Review Checklist:

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Documentation (check at least one)

• There is nothing that needs documenting
• Documentation changes are in this PR
• There is an issue open for the documentation (link?)

Installers

• There is a chance this will affect the installers, if so
  • Windows installer (GH artifact) has been tested (installed and worked)
  • MacOSX installer (GH artifact) has been tested (installed and worked)
  • Wheels installer (GH artifact) has been tested (installed and worked)

Licensing (untick if necessary)

• The introduced changes comply with SasView license (BSD 3-Clause)

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