bump to 5.21.1

[fbergmann]

Description

bumps version, so that a new release of python binaries can be checked.

Motivation and Context

• linux binaries in 5.21.0 would not run with python -Werror

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Change in documentation
• new version for new binaries

Checklist:

• I have updated all documentation necessary.
• I have checked spelling in (new) comments.

Testing

• Testing is done automatically and codecov shows test coverage
• This cannot be tested automatically