error in test_sn_cn_dn_identities

[mario]

In https://groups.google.com/forum/?fromgroups=#!topic/sympy/JEwi4ohGB90 it was reported an error in
`test_sn_cn_dn_identities`;
here is a value for which it fails, found putting `N=20000` in `test_sn_cn_dn_identities`

```
>>> from mpmath import *
>>> mp.dps = 100
>>> jcn = ellipfun('cn')
>>> m = mpf('0.999433209721')
>>> k = m.sqrt()
>>> K = ellipk(k**2)
>>> r = jcn(K, m)
>>> r.ae(0)
False
```

Decreasing `mp.prec`  by 3 in `equality.ae(0)`  the test with `N=20000` passes.