[examples] pep8 fix E26*

E261	at least two spaces before inline comment
E262	inline comment should start with ‘# ‘
E265	block comment should start with ‘# ‘
E266	too many leading ‘#’ for block comment

This PR also fixes some E211 (whitespace before ‘(‘ ) errors overseen by the autopep8/pep8 tool.
@WeatherGod comment here https://github.com/matplotlib/matplotlib/pull/3425#discussion_r16926933 is the reason why I remove spaces between # and code