bpo-38870: Don't start generated output with newlines

[isidentical]

This is a small nit that can cause too much unnecessary nose when using ast.unparse for comparisons. Examples

 $ python
Python 3.9.0a5+ (heads/master:5565c30f0b, Apr 17 2020, 16:56:39) 
[GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import ast
>>> ast.unparse(ast.parse("compile"))
'\ncompile'
>>> ast.unparse(ast.parse("compile")) == "compile"
False
>>> ast.unparse(ast.parse("class compile: pass"))
'\n\nclass compile():\n    pass'

https://bugs.python.org/issue38870

[pablogsal]

Just a nit: I find newline() a bit misleading if it can actually not produce a newline. Maybe rename to maybe_newline or something like that. Otherwise LGTM

[isidentical]

@pablogsal okey, I wasn't sure if we wanted to expose that in the method name but changed it to maybe_newline anyways.