Remove QTextCodec

[jitseniesen]

The plugin uses QTextCodec to read the output of the test process. According to spyder-ide/spyder#20201, this blocks supporting Qt6, so we need to remove it. The PR spyder-ide/spyder#9652 may show how.

[mgorny]

Is it intentional that QTextCodec is still imported here:

spyder-unittest/spyder_unittest/backend/runnerbase.py

Lines 18 to 19 in f716ff1

	from qtpy.QtCore import (
	QObject, QProcess, QProcessEnvironment, QTextCodec, Signal)

?

[jitseniesen]

No, it's not, that is a mistake. Thanks for bringing it to my attention.