Allow empty lines at beginnings of more blocks

Fixes psf#4043, fixes psf#619

These include nested functions and methods.

I think the nested function case quite clearly improves readability. I
think the method case improves consistency, adherence to PEP 8 and
resolves a point of contention.

src/black/lines.py

@@ -738,7 +738,10 @@ def _maybe_empty_lines_for_class_or_def(  # noqa: C901
         if self.previous_line.depth < current_line.depth and (
             self.previous_line.is_class or self.previous_line.is_def
         ):
-            return 0, 0
+            if self.mode.is_pyi or not Preview.allow_empty_first_line_in_block:
+                return 0, 0
+            else:
+                return 1 if user_had_newline else 0, 0
 
         comment_to_add_newlines: Optional[LinesBlock] = None
         if (

tests/data/cases/class_blank_parentheses.py

@@ -39,6 +39,7 @@ def test_func(self):
 
 
 class ClassWithEmptyFunc(object):
+
     def func_with_blank_parentheses():
         return 5
 

tests/data/cases/preview_allow_empty_first_line.py

@@ -22,7 +22,7 @@ def foo():
             Long comment here
             """
             a = 123
-    
+
     if z:
 
         for _ in range(100):
@@ -62,6 +62,15 @@ def method(self):
 
         pass
 
+
+def top_level(
+    a: int,
+    b: str,
+) -> Whatever[Generic, Something]:
+
+    def nested(x: int) -> int:
+        pass
+
 # output
 
 def foo():
@@ -123,6 +132,16 @@ def quux():
 
 
 class Cls:
+
     def method(self):
 
         pass
+
+
+def top_level(
+    a: int,
+    b: str,
+) -> Whatever[Generic, Something]:
+
+    def nested(x: int) -> int:
+        pass

tests/data/cases/preview_form_feeds.py

@@ -37,7 +37,7 @@
 #
 
 #
-        
+
 #
 
 \
@@ -72,7 +72,7 @@
 pass
 
 pass
-        
+
 pass
 
 
@@ -95,7 +95,7 @@ class Baz:
     def __init__(self):
         pass
 
-    
+
     def something(self):
         pass
 
@@ -203,6 +203,7 @@ def bar(a=1, b: bool = False):
 
 
 class Baz:
+
     def __init__(self):
         pass