update CLANG_ALLOW_NON_MODULAR_INCLUDES_IN_FRAMEWORK_MODULES to YES

[frogcjn]

Set project's build settings CLANG_ALLOW_NON_MODULAR_INCLUDES_IN_FRAMEWORK_MODULES = NO.

This change will allow modular framework import non modular headers.

See: beeware/Python-Apple-support#251

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]

See this thread for an ongoing discussion about whether this is an acceptable solution to the underlying problem.