comparators/directory.py: fix whitespace for black 21.12

On black-21.12 test fails as:

    --- diffoscope/comparators/directory.py     2022-01-26 11:54:10.953755 +0000
    +++ diffoscope/comparators/directory.py     2022-01-26 11:54:18.335859 +0000
    @@ -46,11 +46,10 @@
                     "%Y-%m-%d %H:%M:%S",
                     "-f",
                     "%Sp %l %Su %Sg %z %Sm %k %b %#Xf",
                     self.path,
                 ]
    -
     else:
         class Stat(Command):

The change drops extra whitespace.

diffoscope/comparators/directory.py

@@ -49,7 +49,6 @@ if os.uname()[0] == "FreeBSD":
                 self.path,
             ]
 
-
 else:
 
     class Stat(Command):

tests/test_source.py

@@ -256,7 +256,7 @@ def black_version():
     ]
 
 
-@skip_unless_tool_is_at_least("black", black_version, "21.4b2")
+@skip_unless_tool_is_at_least("black", black_version, "21.12b0")
 def test_code_is_black_clean():
     output = subprocess.check_output(
         ("black", "--diff", "."), stderr=subprocess.PIPE