Make wspr.py compatible with Python 3

[kholia]

Before this change:

wspr.py:373: DeprecationWarning: The binary mode of fromstring is deprecated, as it behaves surprisingly on unicode inputs. Use frombuffer instead
  zz = numpy.fromstring(z, numpy.int16)
Traceback (most recent call last):
  File "/home/user/weakmon/./wspr.py", line 1641, in <module>
    main()
  File "/home/user/weakmon/./wspr.py", line 1620, in main
    r.gowav(filename, 0)
  File "/home/user/weakmon/./wspr.py", line 394, in gowav
    self.process(samples, 0)
  File "/home/user/weakmon/./wspr.py", line 563, in process
    dec = self.process1(samples_minute, ss[0:162], hza, noise)
  File "/home/user/weakmon/./wspr.py", line 1219, in process1
    [ msgbits, metric ] = nfano_decode(sym0, sym1)
  File "/home/user/weakmon/./wspr.py", line 139, in nfano_decode
    out_array_type = c_ubyte * (len(in0) / 2)
TypeError: can't multiply sequence by non-int of type 'float'

After this change:

$ python3 ./wspr.py -file ~/Downloads/150426_0918.wav  
1487.2 1.6 0.1 -7 WD4LHT EL89 30
1584.7 1.8 -0.0 -11  W3HH  EL89 30
1444.4 2.1 0.0 -9 ND6P   DM04 30
1515.3 1.4 -0.0 -21 KI7CI  DM09 37
1458.3 1.1 -0.0 -16  W5BIT EL09 17
1527.5 -1.0 0.0 -20 DJ6OL  JO52 37
1500.7 0.3 -0.0 -5 NM7J   DM26 30
1502.1 2.4 -1.0 -14 N2Q/U32ODA 10

The WSPR decoder seems to be functional with Python 3 after this change.

These changes were tested on Ubuntu 22.04 LTS 64-bit OS.