remove rust from qemu dependencies

[Vitaly Shevtsov <shev.vt1984%gmail.com@localhost>]

Hello!

I found that qemu installs rust as a dependency for no vital reason.
It happens because of /usr/pkgsrc/devel/py_request package which
contains this string:
PYTHON_VERSIONED_DEPENDENCIES+= cryptography

I commented this out and qemu still works perfectly with no side effects.
Can we make this optional? Something like this:

.if !empty(PKG_OPTIONS:Rust)
PYTHON_VERSIONED_DEPENDENCIES+= cryptography
.endif

This will make it possible to install qemu without rust dependency.
__
Thanks