Fix iOS12 for black screen

[idbeny]

Modify toga_iOS/libs/uikit.py at line 273.

To fixed iOS12 black screen when launch.

#2262

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

The change looks like exactly what is needed; however the PR is failing CI because it is missing a changenote

In this case, we need a file named changes/2265.bugfix.rst that says something like "Font handling on older versions of iOS has been corrected."

[idbeny]

The change looks like exactly what is needed; however the PR is failing CI because it is missing a changenote

In this case, we need a file named changes/2265.bugfix.rst that says something like "Font handling on older versions of iOS has been corrected."

Sorry, I was so careless.

[freakboy3742]

I've made a minor tweak to correct a linting issue (we require all files to have a trailing newline) - but otherwise, this is great! Thanks for the report and fix!