lint: specify encoding when using 'open'

[umarcor]

Apart from the f-string issues, pylint is complaining about using open without specifying the encoding:

 ************* Module vunit.ui
vunit/ui/__init__.py:251:13: W1514: Using open without explicitly specifying an encoding (unspecified-encoding)
vunit/ui/__init__.py:831:13: W1514: Using open without explicitly specifying an encoding (unspecified-encoding)
************* Module vunit.test.runner
vunit/test/runner.py:310:13: W1514: Using open without explicitly specifying an encoding (unspecified-encoding)

See:

https://pylint.pycqa.org/en/latest/technical_reference/features.html
unspecified-encoding (W1514)

Using open without explicitly specifying an encoding It is better to specify an encoding when opening documents. Using the system default implicitly can create problems on other operating systems. See https://www.python.org/dev/peps/pep-0597/

In this PR, I set the encoding to utf-8 (because of https://www.python.org/dev/peps/pep-0597/#prepare-to-change-the-default-encoding-to-utf-8).