fixture `testdir.makepyfile` does not encode files properly

[d-chris]

seems that testdir.makepyfile() ignores the encoding parameter all together.

FAILED tests/test_makepyfile.py::test_compare_make_basic_file[cp1252-stars] - Failed: testdir.makepyfile encoding ('☆', '☁') with encoding='cp1252', but it sould have failed.
FAILED tests/test_makepyfile.py::test_compare_make_basic_file[cp1252-check-explosion] - Failed: testdir.makepyfile encoding ('✅', '💥') with encoding='cp1252', but it sould have failed. 
FAILED tests/test_makepyfile.py::test_compare_make_basic_file[cp1252-cyrillic_basic] - Failed: testdir.makepyfile encoding ('Я', 'ж') with encoding='cp1252', but it sould have failed.   
FAILED tests/test_makepyfile.py::test_compare_make_basic_file[cp1252-cyrillic_complex] - Failed: testdir.makepyfile encoding ('Ф', 'Щ') with encoding='cp1252', but it sould have failed. 
FAILED tests/test_makepyfile.py::test_compare_make_basic_file[cp1252-chinese_country] - Failed: testdir.makepyfile encoding ('中', '国') with encoding='cp1252', but it sould have failed.
FAILED tests/test_makepyfile.py::test_compare_make_basic_file[cp1252-chinese_concept] - Failed: testdir.makepyfile encoding ('道', '德') with encoding='cp1252', but it sould have failed.

i'll discovered this issue while working on #284