not honouring ~/.rpmmacros

In my `~/.rpmmacros` I have:

```
%_topdir /home/brian/rpm
```

but `pyp2rpm` doesn't seem to be honouring that though:

```
$ pyp2rpm requests-unixsocket
[Errno 2] No such file or directory: '/home/brian/rpmbuild/SOURCES/requests-unixsocket-0.1.5.tar.gz'
```

If I work-around that with `ln -s ../rpm/SOURCES /home/brian/rpmbuild/` things work better:

```
$ pyp2rpm requests-unixsocket >/dev/null
```