Reduce number of goroutines in tests that became flaky after Go 1.14 …

…migration (uber#248)

* Run less go routines in unit tests

* Change number

* Change number

Support for pagination in list catalog (uber#201)

+ Tagserver now sends list and listRepositories in paginated format
+ TagClient support list and listRepositories with pagination
+ registryoverride support pagination for catalog listing
+ Test cases

Update README.md

Attempt to fix flakey announce test (uber#112)

Update pyyaml (uber#68)

due to security vulnerability