scripts: Fixed CsvInt(CsvFloat(mt.inf)) bypassing int/float cast

Turns out mt.isinf is happy to accept non-primitive floats (such as
CsvFloat) as long as __float__ is defined. But CsvInt expects a float
inf, not a CsvFloat, so things explode later.

Fixed by explicitly casting to float if mt.isinf, instead of passing
as-is.

Also tweaked CsvInt/CsvFloat constructors to not bother checking
isinstance, unconditional int/float casts are probably cheaper than the
condition in Python.

scripts/perfbd.py

@@ -52,9 +52,7 @@ class CsvInt(co.namedtuple('CsvInt', 'a')):
                     a = -mt.inf
                 else:
                     raise
-        if not (isinstance(a, int) or mt.isinf(a)):
-            a = int(a)
-        return super().__new__(cls, a)
+        return super().__new__(cls, float(a) if mt.isinf(a) else int(a))
 
     def __repr__(self):
         return '%s(%r)' % (self.__class__.__name__, self.a)