py: Remove unnecessary extra handling of padding of nan/inf.

C's printf will pad nan/inf differently to CPython.  Our implementation
originally conformed to C, now it conforms to CPython's way.

Tests for this are also added in this patch.

tests/float/string_format.py

@@ -23,6 +23,10 @@ test("{:10.4F}", -123.456)
 test("{:10.4G}", 123.456)
 test("{:10.4G}", -123.456)
 
+test("{:06e}", float("inf"))
+test("{:06e}", float("-inf"))
+test("{:06e}", float("nan"))
+
 # The following fails right now
 #test("{:10.1}", 0.0)