`context.bits` will not automatically switch when setting `context.arch` twice.

> I was using IPython to do some quick poc work that time, first I switched `context.arch` to `'amd64'` and assembled `cdqe` and then I suddenly would like to try what if I assemble `cdqe` in 32-bit mode. So I switched `context.arch` back to `'i386'` and wanted to assemble, only to find an error: `AttributeError: Invalid arch/bits combination: i386/64`

Pwntools may can not handle `context.arch` correctly, it could change `context.bits` accordingly when first setting it. However, when setting `context.arch` the second time, `context.bits` left unchanged. I guess this bug is related to `self._tls`?

poc:


one more poc:

```py
from pwn import *
print(f"{context.arch}/{context.bits}")
context.arch = 'amd64'
print(f"{context.arch}/{context.bits}")
context.arch = 'i386'
print(f"{context.arch}/{context.bits}")
```

Start by running the two Python reproductions in the issue and inspect the handling of context.arch and context.bits, including the suspected self._tls state. Done means switching from amd64 to i386 updates the displayed bits to 32 and allows cdqe-related assembly without an invalid arch/bits error.

Written by the indexing model from the issue text.