tests: temporary pin pip to fix the Python 2.7 job

tests/requirements.txt

@@ -10,3 +10,4 @@ coverage>=4.5.1
 pytest-cov>=2.5.1
 paver; python_version>="3.6"
 futures; python_version=="2.7"
+pip==18.1  # Temporary workaround for #1644.

tox.ini

@@ -12,7 +12,7 @@ deps=-rtests/requirements.txt
 # Changed from default (`python -m pip ...`)
 # to prevent the current working directory
 # from being added to `sys.path`.
-install_command={envbindir}/pip install {opts} {packages}
+install_command=python -c 'import sys; sys.path.remove(""); from pkg_resources import load_entry_point; load_entry_point("pip", "console_scripts", "pip")()' install {opts} {packages}
 # Same as above.
 list_dependencies_command={envbindir}/pip freeze
 setenv=COVERAGE_FILE={toxworkdir}/.coverage.{envname}