Fix crash with f-string docstrings

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 "b" in prefix or "B" in prefix or "f" in prefix or "F" in prefix:
         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)}"""