bpo-42997: Improve error message for missing : before suites (GH-24292)

* Add to the peg generator a new directive ('&&') that allows to expect
  a token and hard fail the parsing if the token is not found. This
  allows to quickly emmit syntax errors for missing tokens.

* Use the new grammar element to hard-fail if the ':' is missing before
  suites.

Lib/test/test_syntax.py

@@ -229,7 +229,7 @@ SyntaxError: cannot assign to function call
 
 >>> with a as b
 Traceback (most recent call last):
-SyntaxError: invalid syntax
+SyntaxError: expected ':'
 
 >>> p = p =
 Traceback (most recent call last):
@@ -331,7 +331,7 @@ SyntaxError: Generator expression must be parenthesized
 >>> class C(x for x in L):
 ...     pass
 Traceback (most recent call last):
-SyntaxError: invalid syntax
+SyntaxError: expected ':'
 
 >>> def g(*args, **kwargs):
 ...     print(args, sorted(kwargs.items()))
@@ -708,6 +708,107 @@ leading to spurious errors.
      ...
    SyntaxError: cannot assign to function call
 
+    Missing ':' before suites:
+
+    >>> def f()
+    ...     pass
+    Traceback (most recent call last):
+    SyntaxError: expected ':'
+
+    >>> class A
+    ...     pass
+    Traceback (most recent call last):
+    SyntaxError: expected ':'
+
+   >>> if 1
+   ...   pass
+   ... elif 1:
+   ...   pass
+   ... else:
+   ...   x() = 1
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> if 1:
+   ...   pass
+   ... elif 1
+   ...   pass
+   ... else:
+   ...   x() = 1
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> if 1:
+   ...   pass
+   ... elif 1:
+   ...   pass
+   ... else
+   ...   x() = 1
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> for x in range(10)
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> while True
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> with blech as something
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> with blech
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> with blech, block as something
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> with blech, block as something, bluch
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> with (blech as something)
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> with (blech)
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> with (blech, block as something)
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> with (blech, block as something, bluch)
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> try
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> try:
+   ...   pass
+   ... except
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
 Make sure that the old "raise X, Y[, Z]" form is gone:
    >>> raise X, Y
    Traceback (most recent call last):
@@ -992,7 +1093,7 @@ def func2():
     finally:
         pass
 """
-        self._check_error(code, "invalid syntax")
+        self._check_error(code, "expected ':'")
 
     def test_invalid_line_continuation_left_recursive(self):
         # Check bpo-42218: SyntaxErrors following left-recursive rules