Fix crash with f-string docstrings (#4019)

Python does not consider f-strings to be docstrings, so we probably
shouldn't be formatting them as such

Fixes #4018

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

CHANGES.md

@@ -13,6 +13,9 @@
 
 - Fix crash on formatting code like `await (a ** b)` (#3994)
 
+- No longer treat leading f-strings as docstrings. This matches Python's behaviour and
+  fixes a crash (#4019)
+
 ### Preview style
 
 - Multiline dictionaries and lists that are the sole argument to a function are now

src/black/nodes.py

@@ -529,7 +529,7 @@ def is_docstring(leaf: Leaf) -> bool:
         return False
 
     prefix = get_string_prefix(leaf.value)
-    if "b" in prefix or "B" in prefix:
+    if set(prefix).intersection("bBfF"):
         return False
 
     if prev_siblings_are(

tests/data/cases/docstring_preview.py

@@ -58,7 +58,8 @@ def docstring_almost_at_line_limit():
 
 
 def docstring_almost_at_line_limit_with_prefix():
-    f"""long docstring................................................................"""
+    f"""long docstring................................................................
+    """
 
 
 def mulitline_docstring_almost_at_line_limit():

tests/data/cases/f_docstring.py

@@ -0,0 +1,20 @@
+def foo(e):
+    f""" {'.'.join(e)}"""
+
+def bar(e):
+    f"{'.'.join(e)}"
+
+def baz(e):
+    F""" {'.'.join(e)}"""
+
+# output
+def foo(e):
+    f""" {'.'.join(e)}"""
+
+
+def bar(e):
+    f"{'.'.join(e)}"
+
+
+def baz(e):
+    f""" {'.'.join(e)}"""