can't open tempfile when using memmap array

[Shiki TAKEUCHI]

When I try to input a big array, I get this err.

```
Using memory-mapped arrays...
Traceback (most recent call last):
.......
  File "/ENVPATH/lib/python3.8/site-packages/swan/pycwt.py", line 234, in cwt_f
    return cwt_a(signal, scales, dt, wavelet, ppd, verbose=verbose)
  File "/ENVPATH/lib/python3.8/site-packages/swan/pycwt.py", line 219, in cwt_a
    W = memsafe_arr((len(scales), siglen), 'complex')
  File "/ENVPATH/lib/python3.8/site-packages/swan/pycwt.py", line 34, in memsafe_arr
    return np.memmap(_tmpfile, dtype=dtype, shape=shape)
  File "/ENVPATH/lib/python3.8/site-packages/numpy/core/memmap.py", line 251, in __new__
    fid.write(b'\0')
TypeError: write() argument must be str, not bytes
```
(I am using Python 3.8.6)

This error is caused by line 33 in 'swan/pycwt.py' and to fix this,

- _tmpfile = tmpf.TemporaryFile('w+')
+ _tmpfile = tmpf.TemporaryFile('wb+')

Please consider merge it.

[A Brazhe (brazhe)]

Thank you for pointing this out, merged.