bpo-33012: Fix invalid function cast warnings with gcc 8 in Argument Clinic. (GH-6748)

Fix invalid function cast warnings with gcc 8
for method conventions different from METH_NOARGS, METH_O and
METH_VARARGS in Argument Clinic generated code.
diff --git a/Modules/cjkcodecs/clinic/multibytecodec.c.h b/Modules/cjkcodecs/clinic/multibytecodec.c.h
index a58bb64..74e45bd 100644
--- a/Modules/cjkcodecs/clinic/multibytecodec.c.h
+++ b/Modules/cjkcodecs/clinic/multibytecodec.c.h
@@ -14,7 +14,7 @@
 "registered with codecs.register_error that can handle UnicodeEncodeErrors.");
 
 #define _MULTIBYTECODEC_MULTIBYTECODEC_ENCODE_METHODDEF    \
-    {"encode", (PyCFunction)_multibytecodec_MultibyteCodec_encode, METH_FASTCALL|METH_KEYWORDS, _multibytecodec_MultibyteCodec_encode__doc__},
+    {"encode", (PyCFunction)(void(*)(void))_multibytecodec_MultibyteCodec_encode, METH_FASTCALL|METH_KEYWORDS, _multibytecodec_MultibyteCodec_encode__doc__},
 
 static PyObject *
 _multibytecodec_MultibyteCodec_encode_impl(MultibyteCodecObject *self,
@@ -52,7 +52,7 @@
 "codecs.register_error that is able to handle UnicodeDecodeErrors.\"");
 
 #define _MULTIBYTECODEC_MULTIBYTECODEC_DECODE_METHODDEF    \
-    {"decode", (PyCFunction)_multibytecodec_MultibyteCodec_decode, METH_FASTCALL|METH_KEYWORDS, _multibytecodec_MultibyteCodec_decode__doc__},
+    {"decode", (PyCFunction)(void(*)(void))_multibytecodec_MultibyteCodec_decode, METH_FASTCALL|METH_KEYWORDS, _multibytecodec_MultibyteCodec_decode__doc__},
 
 static PyObject *
 _multibytecodec_MultibyteCodec_decode_impl(MultibyteCodecObject *self,
@@ -89,7 +89,7 @@
 "\n");
 
 #define _MULTIBYTECODEC_MULTIBYTEINCREMENTALENCODER_ENCODE_METHODDEF    \
-    {"encode", (PyCFunction)_multibytecodec_MultibyteIncrementalEncoder_encode, METH_FASTCALL|METH_KEYWORDS, _multibytecodec_MultibyteIncrementalEncoder_encode__doc__},
+    {"encode", (PyCFunction)(void(*)(void))_multibytecodec_MultibyteIncrementalEncoder_encode, METH_FASTCALL|METH_KEYWORDS, _multibytecodec_MultibyteIncrementalEncoder_encode__doc__},
 
 static PyObject *
 _multibytecodec_MultibyteIncrementalEncoder_encode_impl(MultibyteIncrementalEncoderObject *self,
@@ -182,7 +182,7 @@
 "\n");
 
 #define _MULTIBYTECODEC_MULTIBYTEINCREMENTALDECODER_DECODE_METHODDEF    \
-    {"decode", (PyCFunction)_multibytecodec_MultibyteIncrementalDecoder_decode, METH_FASTCALL|METH_KEYWORDS, _multibytecodec_MultibyteIncrementalDecoder_decode__doc__},
+    {"decode", (PyCFunction)(void(*)(void))_multibytecodec_MultibyteIncrementalDecoder_decode, METH_FASTCALL|METH_KEYWORDS, _multibytecodec_MultibyteIncrementalDecoder_decode__doc__},
 
 static PyObject *
 _multibytecodec_MultibyteIncrementalDecoder_decode_impl(MultibyteIncrementalDecoderObject *self,
@@ -280,7 +280,7 @@
 "\n");
 
 #define _MULTIBYTECODEC_MULTIBYTESTREAMREADER_READ_METHODDEF    \
-    {"read", (PyCFunction)_multibytecodec_MultibyteStreamReader_read, METH_FASTCALL, _multibytecodec_MultibyteStreamReader_read__doc__},
+    {"read", (PyCFunction)(void(*)(void))_multibytecodec_MultibyteStreamReader_read, METH_FASTCALL, _multibytecodec_MultibyteStreamReader_read__doc__},
 
 static PyObject *
 _multibytecodec_MultibyteStreamReader_read_impl(MultibyteStreamReaderObject *self,
@@ -309,7 +309,7 @@
 "\n");
 
 #define _MULTIBYTECODEC_MULTIBYTESTREAMREADER_READLINE_METHODDEF    \
-    {"readline", (PyCFunction)_multibytecodec_MultibyteStreamReader_readline, METH_FASTCALL, _multibytecodec_MultibyteStreamReader_readline__doc__},
+    {"readline", (PyCFunction)(void(*)(void))_multibytecodec_MultibyteStreamReader_readline, METH_FASTCALL, _multibytecodec_MultibyteStreamReader_readline__doc__},
 
 static PyObject *
 _multibytecodec_MultibyteStreamReader_readline_impl(MultibyteStreamReaderObject *self,
@@ -338,7 +338,7 @@
 "\n");
 
 #define _MULTIBYTECODEC_MULTIBYTESTREAMREADER_READLINES_METHODDEF    \
-    {"readlines", (PyCFunction)_multibytecodec_MultibyteStreamReader_readlines, METH_FASTCALL, _multibytecodec_MultibyteStreamReader_readlines__doc__},
+    {"readlines", (PyCFunction)(void(*)(void))_multibytecodec_MultibyteStreamReader_readlines, METH_FASTCALL, _multibytecodec_MultibyteStreamReader_readlines__doc__},
 
 static PyObject *
 _multibytecodec_MultibyteStreamReader_readlines_impl(MultibyteStreamReaderObject *self,
@@ -418,4 +418,4 @@
 
 #define _MULTIBYTECODEC___CREATE_CODEC_METHODDEF    \
     {"__create_codec", (PyCFunction)_multibytecodec___create_codec, METH_O, _multibytecodec___create_codec__doc__},
-/*[clinic end generated code: output=2fa0a38494716b97 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=4c1dc8015ee5abb4 input=a9049054013a1b77]*/