21cmSense v2: A modular, open-source 21 cm sensitivity calculator

The 21cm line of neutral hydrogen is a powerful probe of the high-redshift universe (Cosmic Dawn and the Epoch of Reionization), with an unprecedented potential to inform us about key processes of early galaxy formation, the first stars and even cosmology and structure formation, via intensity mapping. It is the subject of a number of current and upcoming low-frequency radio experiments. This paper presents 21cmSense v2.0, which is a Python package that provides a modular framework for calculating the sensitivity of these experiments, in order to enhance the process of their design and forecasting their power for parameter inference. Version 2.0 of 21cmSense has been re-written from the ground up to be more modular and extensible than its venerable predecessor (Pober et al., 2013, 2014), and to provide a more user-friendly interface. The package is freely available both to use and contribute towards at https://github.com/rasg-affiliates/21cmSense.