closes bpo-39605: Fix some casts to not cast away const. (GH-18453)

gcc -Wcast-qual turns up a number of instances of casting away constness of pointers. Some of these can be safely modified, by either:

Adding the const to the type cast, as in:

-    return _PyUnicode_FromUCS1((unsigned char*)s, size);
+    return _PyUnicode_FromUCS1((const unsigned char*)s, size);

or, Removing the cast entirely, because it's not necessary (but probably was at one time), as in:

-    PyDTrace_FUNCTION_ENTRY((char *)filename, (char *)funcname, lineno);
+    PyDTrace_FUNCTION_ENTRY(filename, funcname, lineno);

These changes will not change code, but they will make it much easier to check for errors in consts

Objects/stringlib/asciilib.h

@@ -18,7 +18,7 @@
 #define STRINGLIB_TODECIMAL      Py_UNICODE_TODECIMAL
 #define STRINGLIB_STR            PyUnicode_1BYTE_DATA
 #define STRINGLIB_LEN            PyUnicode_GET_LENGTH
-#define STRINGLIB_NEW(STR,LEN)   _PyUnicode_FromASCII((char*)(STR),(LEN))
+#define STRINGLIB_NEW(STR,LEN)   _PyUnicode_FromASCII((const char*)(STR),(LEN))
 #define STRINGLIB_CHECK          PyUnicode_Check
 #define STRINGLIB_CHECK_EXACT    PyUnicode_CheckExact
 

Objects/stringlib/codecs.h

@@ -46,7 +46,7 @@ STRINGLIB(utf8_decode)(const char **inptr, const char *end,
                     /* Read a whole long at a time (either 4 or 8 bytes),
                        and do a fast unrolled copy if it only contains ASCII
                        characters. */
-                    unsigned long value = *(unsigned long *) _s;
+                    unsigned long value = *(const unsigned long *) _s;
                     if (value & ASCII_CHAR_MASK)
                         break;
 #if PY_LITTLE_ENDIAN
@@ -515,7 +515,7 @@ STRINGLIB(utf16_decode)(const unsigned char **inptr, const unsigned char *e,
             /* Fast path for runs of in-range non-surrogate chars. */
             const unsigned char *_q = q;
             while (_q < aligned_end) {
-                unsigned long block = * (unsigned long *) _q;
+                unsigned long block = * (const unsigned long *) _q;
                 if (native_ordering) {
                     /* Can use buffer directly */
                     if (block & FAST_CHAR_MASK)

Objects/stringlib/find_max_char.h

@@ -28,7 +28,7 @@ STRINGLIB(find_max_char)(const STRINGLIB_CHAR *begin, const STRINGLIB_CHAR *end)
             /* Help register allocation */
             const unsigned char *_p = p;
             while (_p < aligned_end) {
-                unsigned long value = *(unsigned long *) _p;
+                unsigned long value = *(const unsigned long *) _p;
                 if (value & UCS1_ASCII_CHAR_MASK)
                     return 255;
                 _p += SIZEOF_LONG;