DOC: rpy2 examples started failing
The rpy2 examples in the documentation started failing recently (http://pandas-docs.github.io/pandas-docs-travis/r_interface.html) with:

```
In [1]: from rpy2.robjects import r, pandas2ri
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-1-79b90b86f23f> in <module>()
----> 1 from rpy2.robjects import r, pandas2ri

~/miniconda3/envs/pandas/lib/python3.6/site-packages/rpy2/robjects/__init__.py in <module>()
     14 import itertools
     15 from datetime import datetime
---> 16 import rpy2.rinterface as rinterface
     17 import rpy2.rlike.container as rlc
     18 

~/miniconda3/envs/pandas/lib/python3.6/site-packages/rpy2/rinterface/__init__.py in <module>()
     48 del(os)
     49 
---> 50 from rpy2.rinterface._rinterface import (baseenv,
     51                                          emptyenv,
     52                                          endr,

ImportError: dlopen: cannot load any more object with static TLS
```

However, I don't see any change in version between a recent working build and the current failing ([this](https://travis-ci.org/pandas-dev/pandas/jobs/358061154) working one uses rpy2 2.9.1-py36mro343h14c3975_0, and [this](https://travis-ci.org/pandas-dev/pandas/jobs/364254345) failing one 2.9.1-py36mro343h14c3975_0, so the same)

Could this be caused by installing another library?


cc @lgautier 
