fix vim plugin for 19.3b0 (#755) (#766)

plugin/black.vim

@@ -98,15 +98,14 @@ if _initialize_black_env():
 def Black():
   start = time.time()
   fast = bool(int(vim.eval("g:black_fast")))
-  line_length = int(vim.eval("g:black_linelength"))
-  mode = black.FileMode.AUTO_DETECT
-  if bool(int(vim.eval("g:black_skip_string_normalization"))):
-    mode |= black.FileMode.NO_STRING_NORMALIZATION
-  if vim.current.buffer.name.endswith('.pyi'):
-    mode |= black.FileMode.PYI
+  mode = black.FileMode(
+    line_length=int(vim.eval("g:black_linelength")),
+    string_normalization=not bool(int(vim.eval("g:black_skip_string_normalization"))),
+    is_pyi=vim.current.buffer.name.endswith('.pyi'),
+  )
   buffer_str = '\n'.join(vim.current.buffer) + '\n'
   try:
-    new_buffer_str = black.format_file_contents(buffer_str, line_length=line_length, fast=fast, mode=mode)
+    new_buffer_str = black.format_file_contents(buffer_str, fast=fast, mode=mode)
   except black.NothingChanged:
     print(f'Already well formatted, good job. (took {time.time() - start:.4f}s)')
   except Exception as exc: