Add a check for the pip version in setup.py

The new build procedure employing the pyproject.toml file relies
on pip being at least v10.0.0, so we test that this requirement
is satisfied in the setup.py or exit with a warning

setup.py

@@ -9,11 +9,12 @@
 ###########################################################################
 import fastentrypoints
 import re
+import sys
+from distutils.version import StrictVersion
 from os import path
 from setuptools import setup, find_packages
 from setup_requirements import install_requires, extras_require
 
-
 if __name__ == '__main__':
     # Get the version number
     aiida_folder = path.split(path.abspath(__file__))[0]
@@ -22,6 +23,21 @@
         match_expr = "__version__[^'\"]+(['\"])([^'\"]+)"
         aiida_version = re.search(match_expr, aiida_init.read()).group(2).strip()
 
+    # Ensure that pip is installed and the version is at least 10.0.0, which is required for the build process
+    try:
+        import pip
+    except ImportError:
+        print 'Could not import pip, which is required for installation'
+        sys.exit(1)
+
+    PIP_REQUIRED_VERSION = '10.0.0'
+    required_version = StrictVersion(PIP_REQUIRED_VERSION)
+    installed_version = StrictVersion(pip.__version__)
+
+    if installed_version < required_version:
+        print 'The installation requires pip>={}, whereas currently {} is installed'.format(required_version, installed_version)
+        sys.exit(1)
+
     bin_folder = path.join(aiida_folder, 'bin')
     setup(
         name='aiida-core',

utils/validate_pyproject.py

@@ -3,20 +3,21 @@
 import click
 import os
 import sys
+import toml
 
 @click.command()
 def validate_pyproject():
     """
     Ensure that the version of reentry in setup_requirements.py and pyproject.toml are identical
     """
-    import setup_requirements
-    import toml
-
     filename_pyproject = 'pyproject.toml'
     filename_requirements = 'setup_requirements.py'
 
     dir_path = os.path.dirname(os.path.realpath(__file__))
     toml_file = os.path.join(dir_path, os.pardir, filename_pyproject)
+    sys.path.append(os.path.join(dir_path, os.pardir))
+
+    import setup_requirements
 
     reentry_requirement = None