open() manifest savefile in binary mode to make it work with Python 3

[imaami]

Fixes the following error:

$ ./vsdownload.py --save-manifest
Fetching https://aka.ms/vs/16/release/channel
Got toplevel manifest for 16.6.4
Loaded installer manifest for 16.6.4
Traceback (most recent call last):
  File "./vsdownload.py", line 505, in <module>
    packages = getPackages(getManifest(args))
  File "./vsdownload.py", line 150, in getManifest
    f.write(manifestdata)
TypeError: write() argument must be str, not bytes

Signed-off-by: Juuso Alasuutari [email protected]

[mstorsjo]

Thanks!