Defining Nested set crashes gpython

In gpython, if we define a nested set, it will lead to crashing.
`
s = {('string', 1), ('string', 2), ('string', 3)}`

Output on go/wasm(https://gpython.org/?wasm):

```
Gpython 3.4.0 running in your browser with go/wasm
>>> s = {('string', 1), ('string', 2), ('string', 3)}
panic: runtime error: hash of unhashable type py.Tuple
goroutine 5 [running]:
github.com/go-python/gpython/py.NewSetFromItems(0xc09a880, 0x3, 0x4, 0x2)
    /home/ncw/go/src/github.com/go-python/gpython/py/set.go:42 +0xc
github.com/go-python/gpython/vm.do_BUILD_SET(0xc074770, 0x3, 0x0, 0x0)
    /home/ncw/go/src/github.com/go-python/gpython/vm/eval.go:996 +0x5
github.com/go-python/gpython/vm.RunFrame(0xc0de0b0, 0x0, 0xc07bd18, 0x0, 0x0)
    /home/ncw/go/src/github.com/go-python/gpython/vm/eval.go:1785 +0x3f
github.com/go-python/gpython/vm.EvalCodeEx(0xc04c200, 0xc062db0, 0xc062db0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
    /home/ncw/go/src/github.com/go-python/gpython/vm/eval.go:2162 +0xa3
github.com/go-python/gpython/vm.Run(0xc062db0, 0xc062db0, 0xc04c200, 0x0, 0x0, 0x0, 0x0, 0x1, 0x9d240, 0xc04c200)
    /home/ncw/go/src/github.com/go-python/gpython/vm/eval.go:2182 +0x2
github.com/go-python/gpython/repl.(*REPL).Run(0xc09a6c0, 0xc09c080, 0x31)
    /home/ncw/go/src/github.com/go-python/gpython/repl/repl.go:99 +0x20
main.main.func1(0xc01e3f0, 0x2, 0x2)
    /home/ncw/go/src/github.com/go-python/gpython/repl/web/main.go:82 +0x4
syscall/js.callbackLoop()
    /opt/go/go1.11/src/syscall/js/callback.go:116 +0x7
created by syscall/js.NewCallback.func1
    /opt/go/go1.11/src/syscall/js/callback.go:40 +0x2
```

output with gopherjs(https://gpython.org/):

```
Gpython 3.4.0 running in your browser with gopherjs
>>> s = {('string', 1), ('string', 2), ('string', 3)}
[USER]: https://gpython.org/gpython.js: n.keyFor is not a function
$ifaceKeyFor@https://gpython.org/gpython.js:4:9630
LL@https://gpython.org/gpython.js:41:324569
BY@https://gpython.org/gpython.js:44:56407
DO@https://gpython.org/gpython.js:44:91592
DU@https://gpython.org/gpython.js:44:107450
DW@https://gpython.org/gpython.js:44:109015
G.ptr.prototype.Run@https://gpython.org/gpython.js:56:3575
$b@https://gpython.org/gpython.js:60:3908
$b@https://gpython.org/gpython.js:59:2625
r@https://gpython.org/gpython.js:4:23443
$runScheduled@https://gpython.org/gpython.js:4:24007
$schedule@https://gpython.org/gpython.js:4:24184
$go@https://gpython.org/gpython.js:4:23907
I/$packages["github.com/gopherjs/gopherwasm/js"]<@https://gpython.org/gpython.js:59:2240
$externalizeFunction/e.$externalizeWrapper@https://gpython.org/gpython.js:4:28925
a@https://cdnjs.cloudflare.com/ajax/libs/jquery.terminal/1.23.2/js/jquery.terminal.min.js:40:82615
k@https://cdnjs.cloudflare.com/ajax/libs/jquery.terminal/1.23.2/js/jquery.terminal.min.js:40:83463
ENTER@https://cdnjs.cloudflare.com/ajax/libs/jquery.terminal/1.23.2/js/jquery.terminal.min.js:40:14915
$e@https://cdnjs.cloudflare.com/ajax/libs/jquery.terminal/1.23.2/js/jquery.terminal.min.js:40:28362
dispatch@https://code.jquery.com/jquery-latest.js:4641:9
add/elemData.handle@https://code.jquery.com/jquery-latest.js:4309:28
```