graphics/py-pytesseract: 0.3.8 update, fix tests

• update to upstream release 0.3.8, switching to Github because PyPI (the "cheese shop") only carries 0.3.7
• restrict tests to the currently built Python flavor, else tox would try and run others it finds, inheriting possible test failures from other Python versions lacking some packages such as pillow or numpy in their install
• test: add py-numpy to test requisites
• test: add py-pandas to test requisites for Python >= 3.8
• test: add --sitepackages to tox invocation so that the tests find py-pillow.