Updates Python 3 install instructions, fixes #710

[AndreMiras]

Refactors Python 2/3 install instructions.
Makes explicit difference between host and target Python.
Both host Python versions are supported seamlessly while target Python
requires different actions depending on the version.

[tshirtman]

Looks good to me!

[tshirtman]

Just a nit picking, it's better if the "-" lines under titles are the same length as the title itself, as it's what ReStructuredText specify, but i'm not sure it'll make a visible difference in output.

[AndreMiras]

Thanks @tshirtman for reviewing. For the - I think I kinda get what you mean. You mean space+parenthesis e.g. ( should be converted to two dashes i.e. -- ? Is this what you meant?
I was also not sure at first so I did a live test and fixed it before doing the pull request.
You can try the end result on the README of my fork/branch here /~https://github.com/AndreMiras/buildozer/tree/feature/ticket710_update_readme_python3_instructions#installing-buildozer-with-target-python-3-crystax
It seems to work for me, give it a try playing with both link/anchor and let me know if you think it's OK.

[tshirtman]

Sorry, i should have been clearer, i mean the dashes need to continue up to the end of the line, (so under the final ":").

Installing Buildozer with target Python 3 (CrystaX):
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[AndreMiras]

Thank you it's super clear now, thanks for the heads-up! I've fixed it, also I definitely prefer markdown even more now 😄