Avoid defining a new type every time getDefaultProperty is called

Part of lhcb/LHCb#397

This change saves 10 seconds on the test configuration I'm using.

To see the effect in a micro benchmark:

In [1]: %%timeit
   ...: class c:
   ...:     pass
   ...: a = c()
   ...:
   ...:
8.34 µs ± 120 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

In [2]: class c:
   ...:     pass
   ...:

In [3]: %%timeit
   ...: a = c()
   ...:
   ...:
146 ns ± 1.16 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)