5G is the new 3GPP technology designed to solve a wide range of requirements. On the one hand, it must be able to support high bit rates and ultra-low latency services, and on the other hand, it should be able to connect a massive amount of devices with loose bandwidth and delay requirements. Network Slicing is a key paradigm in 5G, and future 6G networks will inherit it for the concurrent provisioning of diverse quality of service. As scheduling is always a delicate vendor topic and there are few free and complete simulation tools to support all 5G features, in this paper, we present Py5cheSim. This is a flexible and open-source simulator based on Python and specially oriented to simulate cell capacity in 3GPP 5G networks and beyond. To the best of our knowledge, Py5cheSim is the first simulator that supports Network Slicing at the Radio Access Network level. It offers an environment that allows the development of new scheduling algorithms in a researcher-friendly way without the need of detailed knowledge of the core of the tool. The present work describes its design and implementation choices, the validation process, the results and different use cases.