python: make Statistics doctests robust to optional ":time" counter (#9729)

The doctests for Statistics.__len__ and Statistics.__getitem__ in
src/api/python/z3/z3.py asserted a fixed counter count (len(st) == 7).
This is fragile because the ":time" entry is only added to statistics
when the elapsed wall-clock time of check() is non-zero (see
collect_timer_stats in src/solver/check_sat_result.h). On fast native
builds, ":time" rounds to 0 and is omitted; under slow environments
(e.g. riscv64 under qemu emulation), it becomes non-zero and is
included, changing len(st).

Fix this by checking the presence of statistics rather than an exact
count.

Signed-off-by: Julien Stephan <jstephan@baylibre.com>

src/api/python/z3/z3.py

@@ -7396,8 +7396,8 @@ class Statistics:
         >>> s.check()
         sat
         >>> st = s.statistics()
-        >>> len(st)
-        7
+        >>> len(st) > 0
+        True
         """
         return int(Z3_stats_size(self.ctx.ref(), self.stats))
 
@@ -7410,8 +7410,8 @@ class Statistics:
         >>> s.check()
         sat
         >>> st = s.statistics()
-        >>> len(st)
-        7
+        >>> len(st) > 0
+        True
         >>> st[0]
         ('nlsat propagations', 2)
         >>> st[1]