Treat bytes objects as immutable in C code #155742

Currently, in the CPython C code, multiple functions treat bytes objects as mutable. For example, they use char* to access their data, instead of const char*.

In the worst case, it can lead to bad bugs like issue gh-155702 in sqlite3 which modifies an immutable bytes object.

Some functions can be rewritten using PyBytesWriter C API to avoid that.

Linked PRs

• gh-155742: Use PyBytesWriter in io _textiowrapper_writeflush() #155743
• gh-155742: Use PyBytesWriter in json _match_number_unicode() #155744
• gh-155742: Use PyBytesWriter in _zstd.train_dict() #155745
• gh-155742: Use PyBytesWriter in CJK codecs #155746
• gh-155742: Use PyBytesWriter in Python/assemble.c #155747
• gh-155742: Use PyBytesWriter in marshal #155748
• gh-155742: Use PyBytesWriter in xml.etree #155749
• gh-155742: Use PyBytesWriter in codecs #155750
• gh-155742: Get PyBytes_AS_STRING() as const char* #155751