Avoid spellchecking the f in an f-string

Before this commit the spellchecker highlights everything in the following:

f'correct word'
^^^^^^^^^

f'wierd word'
^^^^^^^

By adding the `matchgroup` directive, the start and end of the region are not
considered when spellchecking. The examples become:

f'correct word'

f'weird word'
  ^^^^^

Author: m-zat

syntax/python.vim

@@ -210,10 +210,10 @@ else
     syn region pythonString   start=+'''+ skip=+\\'+ end=+'''+ keepend contains=pythonBytesEscape,pythonBytesEscapeError,pythonUniEscape,pythonUniEscapeError,pythonDocTest,pythonSpaceError,@Spell
     syn region pythonString   start=+"""+ skip=+\\"+ end=+"""+ keepend contains=pythonBytesEscape,pythonBytesEscapeError,pythonUniEscape,pythonUniEscapeError,pythonDocTest2,pythonSpaceError,@Spell
 
-    syn region pythonFString   start=+[fF]'+ skip=+\\\\\|\\'\|\\$+ excludenl end=+'+ end=+$+ keepend contains=pythonBytesEscape,pythonBytesEscapeError,pythonUniEscape,pythonUniEscapeError,@Spell
-    syn region pythonFString   start=+[fF]"+ skip=+\\\\\|\\"\|\\$+ excludenl end=+"+ end=+$+ keepend contains=pythonBytesEscape,pythonBytesEscapeError,pythonUniEscape,pythonUniEscapeError,@Spell
-    syn region pythonFString   start=+[fF]'''+ skip=+\\'+ end=+'''+ keepend contains=pythonBytesEscape,pythonBytesEscapeError,pythonUniEscape,pythonUniEscapeError,pythonDocTest,pythonSpaceError,@Spell
-    syn region pythonFString   start=+[fF]"""+ skip=+\\"+ end=+"""+ keepend contains=pythonBytesEscape,pythonBytesEscapeError,pythonUniEscape,pythonUniEscapeError,pythonDocTest2,pythonSpaceError,@Spell
+    syn region pythonFString matchgroup=pythonString  start=+[fF]'+ skip=+\\\\\|\\'\|\\$+ excludenl end=+'+ end=+$+ keepend contains=pythonBytesEscape,pythonBytesEscapeError,pythonUniEscape,pythonUniEscapeError,@Spell
+    syn region pythonFString matchgroup=pythonString  start=+[fF]"+ skip=+\\\\\|\\"\|\\$+ excludenl end=+"+ end=+$+ keepend contains=pythonBytesEscape,pythonBytesEscapeError,pythonUniEscape,pythonUniEscapeError,@Spell
+    syn region pythonFString matchgroup=pythonString  start=+[fF]'''+ skip=+\\'+ end=+'''+ keepend contains=pythonBytesEscape,pythonBytesEscapeError,pythonUniEscape,pythonUniEscapeError,pythonDocTest,pythonSpaceError,@Spell
+    syn region pythonFString matchgroup=pythonString  start=+[fF]"""+ skip=+\\"+ end=+"""+ keepend contains=pythonBytesEscape,pythonBytesEscapeError,pythonUniEscape,pythonUniEscapeError,pythonDocTest2,pythonSpaceError,@Spell
 endif
 
 if s:Python2Syntax()