Bump release version for the development of 5.1.1

[aaronayres35]

[targeting maint/5.1]

This PR bumps the release version and flips the IS_RELEASE flag for the development of the next bug fix release.

Checklist
- [ ] Add a news fragment if this PR is news-worthy for end users. (see docs/releases/README.rst)

[aaronayres35]

Appveyor CI is failing with:

Unexpected error: ConnectionError(MaxRetryError("HTTPSConnectionPool(host='enthought-platform-eds.s3.amazonaws.com', port=443): Max retries exceeded with url: /31cdf7207fdbf37fda85860416ae9d06?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAVEKSXEUY7WUWMADQ%2F20210113%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20210113T184648Z&X-Amz-Expires=604800&X-Amz-SignedHeaders=host&X-Amz-Signature=047071d563f99bf0bd7f24843909759838657bca5845cb504de73577ad6447de (Caused by ProtocolError('Connection aborted.', RemoteDisconnected('Remote end closed connection without response',)))",),)
The traceback has been recorded into the file '/var/folders/5s/g225f6nd6jl4g8tshbh1ltk40000gn/T/tmpth6vk654'. Please send or copy/paste the content of that file when reporting this issue
Command exited with code 255
Build failed

[rahulporuri]

LGTM