ntrip module not working

In a clean Ubuntu 18.04 docker container with mavproxy installed via pip, I get the following error when trying to load the ntrip module:
```

MAV> module load ntrip
MAV> Traceback (most recent call last):
File "/usr/local/bin/mavproxy.py", line 559, in import_package
mod = __import__(name)
File "/usr/local/lib/python3.6/dist-packages/MAVProxy/modules/mavproxy_ntrip.py", line 8, in
from MAVProxy.modules.lib import ntrip
File "/usr/local/lib/python3.6/dist-packages/MAVProxy/modules/lib/ntrip.py", line 13, in
import rtcm3
ModuleNotFoundError: No module named 'rtcm3'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/local/bin/mavproxy.py", line 405, in load_module
m = import_package(modpath)
File "/usr/local/bin/mavproxy.py", line 562, in import_package
mod = __import__(name)
File "/usr/local/lib/python3.6/dist-packages/MAVProxy/modules/mavproxy_ntrip.py", line 8, in
from MAVProxy.modules.lib import ntrip
File "/usr/local/lib/python3.6/dist-packages/MAVProxy/modules/lib/ntrip.py", line 13, in
import rtcm3
ModuleNotFoundError: No module named 'rtcm3'

Traceback (most recent call last):
File "/usr/local/bin/mavproxy.py", line 559, in import_package
mod = __import__(name)
ModuleNotFoundError: No module named 'ntrip'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/local/bin/mavproxy.py", line 405, in load_module
m = import_package(modpath)
File "/usr/local/bin/mavproxy.py", line 562, in import_package
mod = __import__(name)
ModuleNotFoundError: No module named 'ntrip'

Failed to load module: No module named 'ntrip'.
```
The rtcm3.py file actually does exist:
```
root@13314e1688b7:/# find / -name rtcm3.py
/usr/local/lib/python3.6/dist-packages/MAVProxy/modules/lib/rtcm3.py

```
Does anyone know what is going wrong here?

Reproduce the failure in a clean Ubuntu 18.04 container with MAVProxy installed via pip, then inspect MAVProxy/modules/lib/ntrip.py and the adjacent rtcm3.py file. Determine why the existing rtcm3.py cannot be imported; done means the ntrip module loads without the reported traceback.

Written by the indexing model from the issue text.