Bug#1118038: lmfit-py: tests fail: type int64 is not JSON serializable

[Drew Parsons]

Source: lmfit-py
Version: 1.3.3-5
Followup-For: Bug #1118038
Control: severity -1 serious

Despite failing tests, 1.3.3-5 migrated to testing anyway,
so raising this bug severity to serious.