v1.6.0

Changes

• Implement support for new type descriptor protocol
  (by @elprans in 47eec19 for #427)

• Sync errors
  (by @elprans in 3bfb574 for #449)

• Don't depend on exact syntax errors in tests
  (by @msullivan in b3ce0c6 for #451)

• Fix an error in string representation of RelativeDuration.
  (by @vpetrovykh in 667da72 for #453)

• Add multirange support.
  (by @vpetrovykh in 15e280e for #452)