gh-121798: Add class method Decimal.from_number() (GH-121801)

It is an alternate constructor which only accepts a single numeric argument.
Unlike to Decimal.from_float() it accepts also Decimal.
Unlike to the standard constructor, it does not accept strings and tuples.

Lib/_pydecimal.py

@@ -582,6 +582,21 @@ class Decimal(object):
 
         raise TypeError("Cannot convert %r to Decimal" % value)
 
+    @classmethod
+    def from_number(cls, number):
+        """Converts a real number to a decimal number, exactly.
+
+        >>> Decimal.from_number(314)              # int
+        Decimal('314')
+        >>> Decimal.from_number(0.1)              # float
+        Decimal('0.1000000000000000055511151231257827021181583404541015625')
+        >>> Decimal.from_number(Decimal('3.14'))  # another decimal instance
+        Decimal('3.14')
+        """
+        if isinstance(number, (int, Decimal, float)):
+            return cls(number)
+        raise TypeError("Cannot convert %r to Decimal" % number)
+
     @classmethod
     def from_float(cls, f):
         """Converts a float to a decimal number, exactly.