gh-96272: Replace `test_source_encoding`'s `test_pep263` with `test_import_encoded_module` from `test_imp` (GH-96275)

Editors don't agree that `test_source_encoding.py` was valid koi8-r, making it
hard to edit that file without the editor breaking it in some way (see gh-96272).

Only one test actually relied on the koi8-r encoding and it was a duplicate of a
test from the deprecated `imp` module's `test_imp`, so here we replace
`test_pep263` with `test_import_encoded_module` stolen from `test_imp` and
set `test_source_encoding.py`'s encoding to utf-8 to make editing it easier
going forward.