Simplify _to_string encoding of Python strs

[jakirkham]

Makes a few improvements to the Cython logic of _to_string:

• Types the input as str: This allows Cython to use CPython API relevant for the specific type (it will check type on entry)
• Do is not None check first: This way when Cython generates additional None checks inside the branch, the compiler can optimize them out
• Use str's encode method: Cython makes a direct CPython call to generate bytes and then extracts the pointer and size for C++ to copy

[wence-]

Thanks John

[wence-]

/merge

[jakirkham]

Thanks Lawrence! 🙏