Fix mpmath version in nightly tests
#775
| Job | Run time |
|---|---|
| 10m 52s | |
| 13m 37s | |
| 12m 49s | |
| 13m 23s | |
| 8m 21s | |
| 11m 22s | |
| 12m 56s | |
| 13m 4s | |
| 13m 19s | |
| 8m 43s | |
| 12m 28s | |
| 12m 58s | |
| 12m 25s | |
| 14m 6s | |
| 10m 4s | |
| 13m 24s | |
| 13m 21s | |
| 11m 31s | |
| 13m 39s | |
| 12m 1s | |
| 4h 4m 23s |