Alpine: Don't install libraries for tkinter

[JayH5]

• tkinter is rarely used in a container
• The Debian slim images also don't include these libraries (and also still build the module)
• Requires >12MB of libraries (x11, freetype...)
• The module is still built, so if needed, the required libraries can be installed and the module should still work

python        2.7-alpine3.7-no-tkinter   de65e3e2a0c5        About a minute ago   57MB
python        3.6-alpine3.7-no-tkinter   682fd4b35b37        12 minutes ago       74.8MB
python        2.7-alpine3.7              93c09abcfe57        6 days ago           69.5MB
python        3.6-alpine3.7              27e79c0fa4d2        12 days ago          87.4MB

[JayH5]

@tianon @yosifkit any chance one of you has a second to look at this and #281?

Thank you 🙏