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/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;