fuzzer: add special-case for multi-line EOF TokenError (#1991)

* Add special-case for multi-line EOF TokenError under Python3.7

* Update conditional check to correspond to original issue description (and include issue hyperlink)

* Add warning and hint regarding replaying the fuzzer code generation

* Include code review suggestion (with modifications for this to follow)

Co-authored-by: Zac Hatfield-Dodds <zac.hatfield.dodds@gmail.com>

* Remove Python version check, since this issue does exist across more recent Python versions than 3.7

* Update explanatory comment

* Add clarification for potentially-ambiguous blib2to3 import

* Update explanatory comment

* Bring comment into consistent format with previous comment

* Revert "Add clarification for potentially-ambiguous blib2to3 import"

This reverts commit 357b7cc03bdb19dd924f1accd428352f4bb44e5c.

Co-authored-by: Zac Hatfield-Dodds <zac.hatfield.dodds@gmail.com>

fuzz.py

@@ -5,10 +5,13 @@
 a coverage-guided fuzzer I'm working on.
 """
 
+import re
+
 import hypothesmith
 from hypothesis import HealthCheck, given, settings, strategies as st
 
 import black
+from blib2to3.pgen2.tokenize import TokenError
 
 
 # This test uses the Hypothesis and Hypothesmith libraries to generate random
@@ -46,6 +49,16 @@ def test_idempotent_any_syntatically_valid_python(
         # able to cope with it.  See issues #970, #1012, #1358, and #1557.
         # TODO: remove this try-except block when issues are resolved.
         return
+    except TokenError as e:
+        if (
+            e.args[0] == "EOF in multi-line statement"
+            and re.search(r"\r?\n\\\r?\n", src_contents) is not None
+        ):
+            # This is a bug - if it's valid Python code, as above, Black should be
+            # able to cope with it.  See issue #1012.
+            # TODO: remove this block when the issue is resolved.
+            return
+        raise
 
     # And check that we got equivalent and stable output.
     black.assert_equivalent(src_contents, dst_contents)