Transforming unsupported types into a set leads to crashing

Transforming invalid types into a set with set() leads to crashing of gpython. For example:

`x = set('abcde')`

or
```

class A: pass
set(A())

```

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

```
Gpython 3.4.0 running in your browser with go/wasm
>>> class A: pass
>>> set(A())
panic: interface conversion: py.Object is *py.Type, not py.Tuple
goroutine 5 [running]:
github.com/go-python/gpython/py.SetNew(0xc051170, 0xc05ec10, 0x1, 0x1, 0x0, 0x0, 0x0, 0x0, 0x109b0006)
    /home/ncw/go/src/github.com/go-python/gpython/py/set.go:63 +0x14
github.com/go-python/gpython/py.(*Type).M__call__(0xc051170, 0xc05ec10, 0x1, 0x1, 0x0, 0x9d401, 0xc061b00, 0x0, 0x0)
    /home/ncw/go/src/github.com/go-python/gpython/py/type.go:380 +0x4
github.com/go-python/gpython/py.Call(0x9d4a0, 0xc051170, 0xc05ec10, 0x1, 0x1, 0x0, 0x9d4a0, 0xc061b00, 0x0, 0x0)
    /home/ncw/go/src/github.com/go-python/gpython/py/internal.go:169 +0x5
github.com/go-python/gpython/vm.callInternal(0x9d4a0, 0xc051170, 0xc05ec10, 0x1, 0x1, 0x0, 0xc0a4420, 0x9d4a0, 0xc061b00, 0x0, ...)
    /home/ncw/go/src/github.com/go-python/gpython/vm/eval.go:1606 +0xd
github.com/go-python/gpython/vm.(*Vm).Call(0xc0744d0, 0x180000000001, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
    /home/ncw/go/src/github.com/go-python/gpython/vm/eval.go:1689 +0x4e
github.com/go-python/gpython/vm.do_CALL_FUNCTION(0xc0744d0, 0x1, 0x0, 0x0)
    /home/ncw/go/src/github.com/go-python/gpython/vm/eval.go:1428 +0x2
github.com/go-python/gpython/vm.RunFrame(0xc0a4420, 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(0xc04c500, 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, 0xc04c500, 0x0, 0x0, 0x0, 0x0, 0x1, 0x9d240, 0xc04c500)
    /home/ncw/go/src/github.com/go-python/gpython/vm/eval.go:2182 +0x2
github.com/go-python/gpython/repl.(*REPL).Run(0xc09a6c0, 0xc01e4d8, 0x8)
    /home/ncw/go/src/github.com/go-python/gpython/repl/repl.go:99 +0x20
main.main.func1(0xc01e4c0, 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
>>> class A:pass
>>> set(A())
[USER]: https://gpython.org/gpython.js: interface conversion: interface is *py.Type, not py.Tuple
$callDeferred@https://gpython.org/gpython.js:4:22511
$panic@https://gpython.org/gpython.js:4:22957
$assertType@https://gpython.org/gpython.js:4:21361
LM@https://gpython.org/gpython.js:41:325455
MH.ptr.prototype.M__call__@https://gpython.org/gpython.js:41:366934
FK@https://gpython.org/gpython.js:41:255589
DN@https://gpython.org/gpython.js:44:82459
EE.ptr.prototype.Call@https://gpython.org/gpython.js:44:87109
DC@https://gpython.org/gpython.js:44:77098
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

```