Improve setuptools.txt

[geryogam]

This PR will make the following changes:

• fix some typos;
• remove the python command from setup.py calls;
• uniformise quotation marks.

[geryogam]

Would you like to review this PR @jaraco?

[jaraco]

Hey @maggyero. How about instead of adding python setup.py everywhere, write one section that explains (or links to an explanation on) how to execute a Python script and to avoid proliferating the otherwise obvious execution semantics? In other words, I'd like to use the minimum text/code that achieves the desired purpose.

The use of python setup.py only works in some scenarios, such as when python points to the preferred interpreter (which it often doesn't, especially in today's split Python 2/3 world). In practice, the user will need to execute python3 setup.py or py -3 setup.py or some other invocation.

Moreover, *.py works on Windows (invoking the default Python interpreter) and ./setup.py works on Unix OSs, assuming the 'executable' bit is set. All of these nuances are difficult to capture in each and every execution, which is why the documentation leaves it as an exercise to the user to figure out the best way to execute the python script, and why simply setup.py is preferable to python setup.py.

How about simply removing the python3 from the `setup.py invocation, and fix the other typos?

[geryogam]

@jaraco

How about simply removing the python3 from the `setup.py invocation, and fix the other typos?

That makes sense. Updated.