Move cpp_exceptions_helper.cpp to cpp_exceptions_helper.h so it is picked up by sdist.

--HG--
rename : tests/run/cpp_exceptions_helper.cpp => tests/run/cpp_exceptions_helper.h

tests/run/cpp_exceptions.pyx

 cdef int raise_py_error() except *:
     raise TypeError("custom")
 
-cdef extern from "cpp_exceptions_helper.cpp":
+cdef extern from "cpp_exceptions_helper.h":
     cdef int raise_int_raw "raise_int"(bint fire) except +
     cdef int raise_int_value "raise_int"(bint fire) except +ValueError
     cdef int raise_int_custom "raise_int"(bint fire) except +raise_py_error