PyPA manifest files are not recognized (#12707)

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

runtime/filetype.vim

@@ -1679,6 +1679,9 @@ au BufNewFile,BufRead *.pk			setf poke
 " Protocols
 au BufNewFile,BufRead */etc/protocols		setf protocols
 
+" PyPA manifest files
+au BufNewFile,BufRead MANIFEST.in		setf pymanifest
+
 " Pyret
 au BufNewFile,BufRead *.arr			setf pyret
 

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'],