libcpp.string supports s[0]= and s.at(0)=

Search for pxd files in sys.path.

FIX: prevent missing cimport warnings when using cythonize.

The `cython` package fakes several sub-modules, including
`cython.operator` and `cython.view`. Since the `cythonize` command
cannot find a corresponding pxd file for these modules, it prints
a warning. To fix this, we exclude all modules in the `cython`
namespace when checking for missing pxd files.