Remove useless chainmap dependency (#2799)

It seems the dependency was introduced together with the caching
mechanism but ended up not being used at all.

.travis.yml

@@ -36,11 +36,8 @@ before_install:
 install:
     # Upgrade pip setuptools and wheel to be able to run the next command
     - pip install -U pip==18.1 wheel setuptools coveralls
-    # Need numpy version that is compatible with pymatgen, see #2420
-    # Specifying it in pyproject.toml is not sufficient for travis (unknown reason)
-    - pip install numpy==1.15.4
     # Install AiiDA with some optional dependencies
-    - if [ "$TEST_TYPE" == "docs" ]; then pip install . && pip install -r docs/requirements_for_rtd.txt; else pip install .[all]; fi
+    - if [ "$TEST_TYPE" == "docs" ]; then pip install --no-cache-dir . && pip install --no-cache-dir -r docs/requirements_for_rtd.txt; else pip install .[all]; fi
 
 env:
     ## Build matrix to test both backends, and the docs

aiida/common/caching.py

@@ -13,10 +13,6 @@
 import copy
 from functools import wraps
 from contextlib import contextmanager
-try:
-    from collections import ChainMap
-except ImportError:
-    from chainmap import ChainMap
 
 import yaml
 from plum.util import load_class

aiida/work/workfunction.py

@@ -67,4 +67,4 @@ def wrapped_function(*args, **kwargs):
                 return results
 
     wrapped_function._is_workfunction = True
-    return wrapped_function
+    return wrapped_function

docs/requirements_for_rtd.txt

@@ -21,7 +21,6 @@ anyjson==0.3.3
 ase==3.12.0
 billiard==3.3.0.23
 celery==3.1.25
-chainmap
 click-plugins==1.0.3
 click-spinner==0.1.7
 click==6.7

setup_requirements.py

@@ -62,7 +62,7 @@
 ]
 
 extras_require = {
-    ':python_version < "3"': ['chainmap', 'singledispatch >= 3.4.0.3'],
+    ':python_version < "3"': ['singledispatch >= 3.4.0.3'],
     # Requirements for ssh transport with authentification through Kerberos token
     # N. B.: you need to install first libffi and MIT kerberos GSSAPI including header files.
     # E.g. for Ubuntu 14.04: sudo apt-get install libffi-dev libkrb5-dev