Fix pyproject.toml formatting error

[GenevieveBuckley]

The cookicutter template is gnerating a pyproject.toml file with two missing \n characters. There should be a line break before [tool.briefcase.app.helloworld.windows] in the generated file.

We can fix this by moving the cookiecutter tags inside the square brackets of our list.
(The cookiecutter tags seem "greedy" and appear to suck up extra whitespace around them.)

Closes beeware/briefcase#405

Before the fix:

{%- if cookiecutter.gui_framework == 'Toga' %}
system_requires = [
    'libgirepository1.0-dev',
    'libcairo2-dev',
    'libpango1.0-dev',
    'libwebkitgtk-3.0-0',
    'gir1.2-webkit-3.0',
]
{% endif -%}

Generates this in pyproject.toml

...
[tool.briefcase.app.helloworld.macOS]
requires = []

[tool.briefcase.app.helloworld.linux]
requires = [][tool.briefcase.app.helloworld.windows]
requires = []

# Mobile deployments
...

After the fix:

The cookicutter tags have been moved inside the square brackets of the list.

system_requires = [
{%- if cookiecutter.gui_framework == 'Toga' %}
    'libgirepository1.0-dev',
    'libcairo2-dev',
    'libpango1.0-dev',
    'libwebkitgtk-3.0-0',
    'gir1.2-webkit-3.0',
{% endif -%}
]

Generates this correct pyproject.toml

[tool.briefcase.app.helloworld.macOS]
requires = []

[tool.briefcase.app.helloworld.linux]
requires = []
system_requires = []

[tool.briefcase.app.helloworld.windows]
requires = []

# Mobile deployments

[freakboy3742]

Looks great! Thanks for the contribution!

For the record - the "greedy" behavior you've observed is a feature of Jinja; {% if ... %} is an if tag, but the - on the tag [controls whether whitespace around the tag are consumed(https://jinja.palletsprojects.com/en/2.11.x/templates/?highlight=newlines#whitespace-control). In this case, it looks like the template was consuming a little too enthusiastically. The update you've provided tackles the problem in a slightly different, but probably better way - ensuring that there is always a system_requires directive, with only the contents being framework-specific.

If you're interested in a stretch goal related to this fix, I'd love to add a test suite to this repository so that bugs of this nature don't occur again in future. I've written up some requirements on #19; if you're interested, and/or if you have any questions, let me know!

Also - next time there's an in-person PyCon AU, don't forget to remind me that you're owed a Challenge Coin :-)