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

Python/ceval.c

@@ -5440,7 +5440,7 @@ dtrace_function_entry(PyFrameObject *f)
     funcname = PyUnicode_AsUTF8(f->f_code->co_name);
     lineno = PyCode_Addr2Line(f->f_code, f->f_lasti);
 
-    PyDTrace_FUNCTION_ENTRY((char *)filename, (char *)funcname, lineno);
+    PyDTrace_FUNCTION_ENTRY(filename, funcname, lineno);
 }
 
 static void
@@ -5454,7 +5454,7 @@ dtrace_function_return(PyFrameObject *f)
     funcname = PyUnicode_AsUTF8(f->f_code->co_name);
     lineno = PyCode_Addr2Line(f->f_code, f->f_lasti);
 
-    PyDTrace_FUNCTION_RETURN((char *)filename, (char *)funcname, lineno);
+    PyDTrace_FUNCTION_RETURN(filename, funcname, lineno);
 }
 
 /* DTrace equivalent of maybe_call_line_trace. */
@@ -5486,7 +5486,7 @@ maybe_dtrace_line(PyFrameObject *frame,
         co_name = PyUnicode_AsUTF8(frame->f_code->co_name);
         if (!co_name)
             co_name = "?";
-        PyDTrace_LINE((char *)co_filename, (char *)co_name, line);
+        PyDTrace_LINE(co_filename, co_name, line);
     }
     *instr_prev = frame->f_lasti;
 }

Python/marshal.c

@@ -734,7 +734,7 @@ r_byte(RFILE *p)
     else {
         const char *ptr = r_string(1, p);
         if (ptr != NULL)
-            c = *(unsigned char *) ptr;
+            c = *(const unsigned char *) ptr;
     }
     return c;
 }

Python/pyhash.c

@@ -366,7 +366,7 @@ static PyHash_FuncDef PyHash_Func = {fnv, "fnv", 8 * SIZEOF_PY_HASH_T,
 static uint64_t
 siphash24(uint64_t k0, uint64_t k1, const void *src, Py_ssize_t src_sz) {
     uint64_t b = (uint64_t)src_sz << 56;
-    const uint8_t *in = (uint8_t*)src;
+    const uint8_t *in = (const uint8_t*)src;
 
     uint64_t v0 = k0 ^ 0x736f6d6570736575ULL;
     uint64_t v1 = k1 ^ 0x646f72616e646f6dULL;

Python/sysmodule.c

@@ -204,7 +204,7 @@ PySys_Audit(const char *event, const char *argFormat, ...)
 
     /* Dtrace USDT point */
     if (dtrace) {
-        PyDTrace_AUDIT((char *)event, (void *)eventArgs);
+        PyDTrace_AUDIT(event, (void *)eventArgs);
     }
 
     /* Call interpreter hooks */