PyPA manifest files are not recognized (#12707)

Problem:    PyPA manifest files are not recognized.
Solution:   Add a pattern to match PyPA manifest files.

src/testdir/test_filetype.vim

@@ -544,6 +544,7 @@ def s:GetFilenameChecks(): dict<list<string>>
     psl: ['file.psl'],
     pug: ['file.pug'],
     puppet: ['file.pp'],
+    pymanifest: ['MANIFEST.in'],
     pyret: ['file.arr'],
     pyrex: ['file.pyx', 'file.pxd'],
     python: ['file.py', 'file.pyw', '.pythonstartup', '.pythonrc', 'file.ptl', 'file.pyi', 'SConstruct'],