GMP website: old release (3.1.1) cannot be downloaded

[Peter Jin]

Hello,

It appears that at least one of the old archived GMP releases (version 3.1.1) cannot be downloaded from the GMP website. This is possibly due to bad permissions on the file (not readable by the web server?).

Steps to reproduce:

1. Go to https://gmplib.org/download/gmp/archive/
2. Try to download either "gmp-3.1.1.tar.bz2" or "gmp-3.1.1.tar.xz"
3. Response: HTTP/2 403 Forbidden (server: nginx/1.18.0, etc.)

Hope this is helpful. Thank you!
-Peter