Merge python3 into master to implement #62

[RedFantom]

The commits in this branch drop support for:

• Python 2.7 and earlier
• Official support for Python 3.4 and earlier, though some builds may still work
• Python versions using Tk 8.5 or earlier
• The python-tkimg package

In addition, the CI files have been updated in order to test on the newest Python versions.

[Akuli]

Why are travis macos things deleted?

[RedFantom]

@Akuli Because the Python version installed with brew only supports Tk 8.5, unfortunately. I don't have a macOS computer and I don't have access to one, so I'm not sure how to test on macOS either. The Travis-CI never ran the full tests for the package anyway, as I couldn't get it to work. It only built a wheel as a very rudimentary test, and it was an allowed failure too.

From the last build logs:

Running system command:  sudo -H $PYTHON setup.py bdist_wheel
Traceback (most recent call last):
  File "setup.py", line 17, in <module>
    raise RuntimeError(message)
RuntimeError: This version of ttkthemes does not support Tk 8.5 and earlier. Please install an earlier version.
Traceback (most recent call last):
  File "ci.py", line 155, in <module>
    ci_macos()
  File "ci.py", line 136, in ci_macos
    assert check_wheel_existence()
  File "ci.py", line 53, in check_wheel_existence
    return len([file for file in os.listdir("dist") if file.endswith((".whl", ".tar.gz"))]) != 0
FileNotFoundError: [Errno 2] No such file or directory: 'dist'

If you have any advice on this or you do experience issues because of the changes, then please let me know by opening an issue report.