[VENDOR] Bump urllib3 to fix CVE-2019-11324

[eoghanmcilwaine]

Bumped package urllib3 from 1.24.1 to 1.24.3 to fix the following security error: https://nvd.nist.gov/vuln/detail/CVE-2019-11324

Also bumped requests from 2.20.0 to 2.21.0 for compatibility.

LRN-23661

Checklist

• [ ] Feature

• [ ] Bug

• [x] Security

• [ ] Documentation

• [x] ChangeLog.md updated

• [ ] Tests added

• [ ] All testsuites passed

• [x] make dist completed successfully

[shtrom]

Looks good. Did you run the test suite and try make dist?

[shtrom]

Also, the ChangeLog.