Importerror: No Module Named Attr

I have only very rudimentary experience in Python. I am trying to install the package pyslim (see here on the pypi website). I did

$ pip install pyslim

Requirement already satisfied: pyslim in ./Library/Python/2.7/lib/python/site-packages/pyslim-0.1-py2.7.egg (0.1)
Requirement already satisfied: msprime in /usr/local/lib/python2.7/site-packages (from pyslim) (0.6.1)
Requirement already satisfied: attrs in /usr/local/lib/python2.7/site-packages (from pyslim) (16.3.0)
Requirement already satisfied: svgwrite in /usr/local/lib/python2.7/site-packages (from msprime->pyslim) (1.1.12)
Requirement already satisfied: jsonschema in /usr/local/lib/python2.7/site-packages (from msprime->pyslim) (2.6.0)
Requirement already satisfied: six in /usr/local/lib/python2.7/site-packages (from msprime->pyslim) (1.10.0)
Requirement already satisfied: numpy>=1.7.0 in /usr/local/lib/python2.7/site-packages/numpy-1.10.4-py2.7-macosx-10.11-x86_64.egg (from msprime->pyslim) (1.10.4)
Requirement already satisfied: h5py in /usr/local/lib/python2.7/site-packages (from msprime->pyslim) (2.8.0)
Requirement already satisfied: pyparsing>=2.0.1 in /usr/local/lib/python2.7/site-packages (from svgwrite->msprime->pyslim) (2.2.0)
Requirement already satisfied: functools32; python_version == "2.7" in /usr/local/lib/python2.7/site-packages (from jsonschema->msprime->pyslim) (3.2.3.post2)

But when I open python and try to import pyslim, it fails

> import pyslim

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/remi/Library/Python/2.7/lib/python/site-packages/", line 4, in <module>
    from pyslim.slim_metadata import *       # NOQA
  File "/Users/remi/Library/Python/2.7/lib/python/site-packages/", line 1, in <module>
    import attr
ImportError: No module named attr

So, I did

$ pip install attr
Requirement already satisfied: attr in /usr/local/lib/python2.7/site-packages (0.3.1)

and

$ pip install attrs
Requirement already satisfied: attrs in /usr/local/lib/python2.7/site-packages (16.3.0)

I restarted python and tried to import pyslim again but I keep receiving the same error message. I also tried to download and install the files from github by doing

$ git clone 
$ cd pyslim
$ python setup.py install --user

as indicated here on the pypi website. On this last line of code, I get a long output ending with

Download error on  [SSL: TLSV1_ALERT_PROTOCOL_VERSION] tlsv1 alert protocol version (_ssl.c:590) -- Some packages may not be found!
No local packages or download links found for attrs
error: Could not find suitable distribution for Requirement.parse('attrs')

I am using Python 2.7.10 on a MAC OSX 10.11.6. Not sure if it matter but I usually install things with homebrew. I am using pip 18.1 from /usr/local/lib/python2.7/site-packages/pip (python 2.7).

Edit

$ which python
   /usr/bin/python
$ which pip
   /usr/local/bin/pip