bpo-45434: Remove pystrhex.h header file (GH-28923)

Move Include/pystrhex.h to Include/internal/pycore_strhex.h.
The header file only contains private functions.

The following C extensions are now built with Py_BUILD_CORE_MODULE
macro defined to get access to the internal C API:

* _blake2
* _hashopenssl
* _md5
* _sha1
* _sha3
* _ssl
* binascii

Include/internal/pycore_strhex.h

@@ -0,0 +1,36 @@
+#ifndef Py_INTERNAL_STRHEX_H
+#define Py_INTERNAL_STRHEX_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef Py_BUILD_CORE
+#  error "this header requires Py_BUILD_CORE define"
+#endif
+
+// Returns a str() containing the hex representation of argbuf.
+PyAPI_FUNC(PyObject*) _Py_strhex(const
+    char* argbuf,
+    const Py_ssize_t arglen);
+
+// Returns a bytes() containing the ASCII hex representation of argbuf.
+PyAPI_FUNC(PyObject*) _Py_strhex_bytes(
+    const char* argbuf,
+    const Py_ssize_t arglen);
+
+// These variants include support for a separator between every N bytes:
+PyAPI_FUNC(PyObject*) _Py_strhex_with_sep(
+    const char* argbuf,
+    const Py_ssize_t arglen,
+    const PyObject* sep,
+    const int bytes_per_group);
+PyAPI_FUNC(PyObject*) _Py_strhex_bytes_with_sep(
+    const char* argbuf,
+    const Py_ssize_t arglen,
+    const PyObject* sep,
+    const int bytes_per_group);
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_INTERNAL_STRHEX_H */