Issue #22221: Backported fixes from Python 3 (issue #18960).

* Now the source encoding declaration on the second line isn't effective if
  the first line contains anything except a comment.  This affects compile(),
  eval() and exec() too.

* IDLE now ignores the source encoding declaration on the second line if the
  first line contains anything except a comment.

* 2to3 and the findnocoding.py script now ignore the source encoding
  declaration on the second line if the first line contains anything except
  a comment.
diff --git a/Lib/lib2to3/pgen2/tokenize.py b/Lib/lib2to3/pgen2/tokenize.py
index 4cb2a41..d64a3e6 100644
--- a/Lib/lib2to3/pgen2/tokenize.py
+++ b/Lib/lib2to3/pgen2/tokenize.py
@@ -237,6 +237,7 @@
             toks_append(tokval)
 
 cookie_re = re.compile(r'^[ \t\f]*#.*coding[:=][ \t]*([-\w.]+)')
+blank_re = re.compile(r'^[ \t\f]*(?:[#\r\n]|$)')
 
 def _get_normal_name(orig_enc):
     """Imitates get_normal_name in tokenizer.c."""
@@ -309,6 +310,8 @@
     encoding = find_cookie(first)
     if encoding:
         return encoding, [first]
+    if not blank_re.match(first):
+        return default, [first]
 
     second = read_or_stop()
     if not second: