Add the PyOS_*() string comparison/conversion functions as cpython/conversion.pxd.

Cython/Includes/cpython/conversion.pxd

+# From https://docs.python.org/3/c-api/conversion.html
+
+from .object cimport PyObject
+
+cdef extern from "Python.h":
+    ctypedef struct va_list
+
+    int PyOS_snprintf(char *str, size_t size, const char *format, ...)
+    # Output not more than size bytes to str according to the format
+    # string format and the extra arguments. See the Unix man page snprintf(2).
+
+    int PyOS_vsnprintf(char *str, size_t size, const char *format, va_list va)
+    # Output not more than size bytes to str according to the format
+    # string format and the variable argument list va. Unix man page vsnprintf(2).
+
+    double PyOS_string_to_double(const char *s, char **endptr, PyObject *overflow_exception)
+    # Convert a string s to a double, raising a Python exception on failure. The set of
+    # accepted strings corresponds to the set of strings accepted by Python’s float()
+    # constructor, except that s must not have leading or trailing whitespace.
+    # The conversion is independent of the current locale.
+
+    char* PyOS_double_to_string(double val, char format_code, int precision, int flags, int *ptype)
+    # Convert a double val to a string using supplied format_code, precision, and flags.
+
+    int PyOS_stricmp(const char *s1, const char *s2)
+    # Case insensitive comparison of strings. The function works almost identically
+    # to strcmp() except that it ignores the case.
+
+    int PyOS_strnicmp(const char *s1, const char *s2, Py_ssize_t  size)
+    # Case insensitive comparison of strings. The function works almost identically
+    # to strncmp() except that it ignores the case.

tests/run/cython_includes.pyx

@@ -15,6 +15,7 @@ cimport cpython.ceval
 cimport cpython.cobject
 cimport cpython.codecs
 cimport cpython.complex
+cimport cpython.conversion
 cimport cpython.datetime
 cimport cpython.dict
 cimport cpython.exc