Global import of external samplings breaks setup 
## Description

Import error of `pyfar.spatial.external` which is a private submodule, not a global module.

Occurs on local install on some systems running `python setup.py install`.

## What I Did

Corresponding traceback of install.

```
File "import_pyfar_test.py", line 1, in <module>
    from pyfar import spatial
  File "/usr/local/Caskroom/miniconda/base/envs/pyfar/lib/python3.8/site-packages/pyfar-0.1.0-py3.8.egg/pyfar/__init__.py", line 15, in <module>
    from . import spatial
  File "/usr/local/Caskroom/miniconda/base/envs/pyfar/lib/python3.8/site-packages/pyfar-0.1.0-py3.8.egg/pyfar/spatial/__init__.py", line 6, in <module>
    from . import samplings
  File "/usr/local/Caskroom/miniconda/base/envs/pyfar/lib/python3.8/site-packages/pyfar-0.1.0-py3.8.egg/pyfar/spatial/samplings.py", line 10, in <module>
    from pyfar.spatial.external import (
ModuleNotFoundError: No module named 'pyfar.spatial.external'
```

diff --git a/pyfar/dsp/filter.py b/pyfar/dsp/filter.py
index 36cac98c..88be8a18 100644
--- a/pyfar/dsp/filter.py
+++ b/pyfar/dsp/filter.py
@@ -3,7 +3,7 @@ import scipy.signal as spsignal
 
 from .classes import (FilterIIR, FilterSOS)
 
-import pyfar.dsp._audiofilter as iir
+from . import _audiofilter as iir
 
 
 def butter(signal, N, frequency, btype='lowpass', sampling_rate=None):
diff --git a/pyfar/spatial/samplings.py b/pyfar/spatial/samplings.py
index 91cd9236..8d0ed35e 100644
--- a/pyfar/spatial/samplings.py
+++ b/pyfar/spatial/samplings.py
@@ -7,8 +7,8 @@ import urllib3
 import os
 import scipy.io as sio
 from pyfar.coordinates import Coordinates
-from pyfar.spatial.external import (
-    samplings_lebedev, eq_area_partitions)
+
+from .external import (samplings_lebedev, eq_area_partitions)
 
 
 def cart_equidistant_cube(n_points):
['pyfar/spatial/samplings.py', 'pyfar/dsp/filter.py']