Based on https://sourceforge.net/p/ruamel-yaml-clib/code/merge-requests/4/ with additions
for `input_handler` and `output_handler`.

--- a/_ruamel_yaml.pxd
+++ b/_ruamel_yaml.pxd
@@ -2,15 +2,15 @@
 cdef extern from "_ruamel_yaml.h":
 
     void malloc(int l)
-    void memcpy(char *d, char *s, int l)
+    void memcpy(unsigned char *d, char *s, int l)
     int strlen(char *s)
     int PyString_CheckExact(object o)
     int PyUnicode_CheckExact(object o)
     char *PyString_AS_STRING(object o)
     int PyString_GET_SIZE(object o)
-    object PyString_FromStringAndSize(char *v, int l)
+    object PyString_FromStringAndSize(unsigned char *v, size_t l)
     object PyUnicode_FromString(char *u)
-    object PyUnicode_DecodeUTF8(char *u, int s, char *e)
+    object PyUnicode_DecodeUTF8(unsigned char *u, size_t s, char *e)
     object PyUnicode_AsUTF8String(object o)
     int PY_MAJOR_VERSION
 
@@ -85,11 +85,11 @@
         YAML_MAPPING_START_EVENT
         YAML_MAPPING_END_EVENT
 
-    ctypedef int yaml_read_handler_t(void *data, char *buffer,
-            int size, int *size_read) except 0
-
-    ctypedef int yaml_write_handler_t(void *data, char *buffer,
-            int size) except 0
+    ctypedef int yaml_read_handler_t(void *data, unsigned char *buffer,
+            size_t size, size_t *size_read) except 0
+
+    ctypedef int yaml_write_handler_t(void *data, unsigned char *buffer,
+            size_t size) except 0
 
     ctypedef struct yaml_mark_t:
         int index
@@ -112,7 +112,7 @@
         char *handle
         char *suffix
     ctypedef struct _yaml_token_scalar_data_t:
-        char *value
+        unsigned char *value
         int length
         yaml_scalar_style_t style
     ctypedef struct _yaml_token_version_directive_data_t:
@@ -151,7 +151,7 @@
     ctypedef struct _yaml_event_scalar_data_t:
         char *anchor
         char *tag
-        char *value
+        unsigned char *value
         int length
         int plain_implicit
         int quoted_implicit
--- a/_ruamel_yaml.pyx
+++ b/_ruamel_yaml.pyx
@@ -904,7 +904,7 @@
                 raise error
         return 1

-cdef int input_handler(void *data, char *buffer, int size, int *read) except 0:
+cdef int input_handler(void *data, unsigned char *buffer, size_t size, size_t *read) except 0:
     cdef CParser parser
     parser = <CParser>data
     if parser.stream_cache is None:
@@ -1514,7 +1514,7 @@
             self.ascend_resolver()
         return 1

-cdef int output_handler(void *data, char *buffer, int size) except 0:
+cdef int output_handler(void *data, unsigned char *buffer, size_t size) except 0:
     cdef CEmitter emitter
     emitter = <CEmitter>data
     if emitter.dump_unicode == 0: