__len__ return type fixed; isinstance() support is better

[swenson]

• __len__ should always return a Python Int
• isinstance should work for more types
• Exception passed to PyErr_SetString is a Python callable, not a JS class
• add random.random

PR Checklist:

• [x] All new features have been tested
• [x] All new features have been documented
• [x] I have read the CONTRIBUTING.md file
• [x] I will abide by the code of conduct

[swenson]

Thanks!