patch 8.2.2271: ml_get error when changing hidden buffer in Python

Problem:    ml_get error when changing hidden buffer in Python.
Solution:   Block updating folds. (closes #7598)

src/testdir/test_python3.vim

@@ -4016,4 +4016,37 @@ func Test_python3_non_utf8_string()
   sunmap <Esc>@
 endfunc
 
+func Test_python3_fold_hidden_buffer()
+  CheckFeature folding
+
+  set fdm=expr fde=Fde(v:lnum)
+  let b:regex = '^'
+  func Fde(lnum)
+    let ld = [{}]
+    let lines = bufnr('%')->getbufline(1, '$')
+    let was_import = 0
+    for lnum in range(1, len(lines))
+      let line = lines[lnum]
+      call add(ld, {'a': b:regex})
+      let ld[lnum].foldexpr = was_import ? 1 : '>1'
+      let was_import = 1
+    endfor
+    return ld[a:lnum].foldexpr
+  endfunc
+
+  call setline(1, repeat([''], 15) + repeat(['from'], 3))
+  eval repeat(['x'], 17)->writefile('Xa.txt')
+  split Xa.txt
+  py3 import vim
+  py3 b = vim.current.buffer
+  py3 aaa = b[:]
+  hide
+  py3 b[:] = aaa
+
+  call delete('Xa.txt')
+  set fdm& fde&
+  delfunc Fde
+  bwipe! Xa.txt
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab