block_string: print multi line for trailing backslash

Replicates graphql/graphql-js@15e43b8

Author: Cito

src/graphql/language/block_string.py

@@ -79,8 +79,12 @@ def print_block_string(
     is_single_line = "\n" not in value
     has_leading_space = value.startswith(" ") or value.startswith("\t")
     has_trailing_quote = value.endswith('"')
+    has_trailing_slash = value.endswith("\\")
     print_as_multiple_lines = (
-        not is_single_line or has_trailing_quote or prefer_multiple_lines
+        not is_single_line
+        or has_trailing_quote
+        or has_trailing_slash
+        or prefer_multiple_lines
     )
 
     # Format a multi-line block quote to account for leading space.

tests/language/test_block_string.py

@@ -120,6 +120,12 @@ def correctly_prints_single_line_with_leading_space_and_quotation():
             == '"""    space-led value "quoted string"\n"""'
         )
 
+    def correctly_prints_single_line_with_trailing_backslash():
+        s = "backslash \\"
+
+        assert print_block_string(s) == '"""\nbackslash \\\n"""'
+        assert print_block_string(s, "", True) == '"""\nbackslash \\\n"""'
+
     def correctly_prints_string_with_a_first_line_indentation():
         s = join_lines("    first  ", "  line     ", "indentation", "     string")