Revert "ci: avoid editable installs with old pip"

This reverts commit 0fe600e.

.circleci/prepare.sh

@@ -5,6 +5,6 @@ set -o xtrace
 $PYTHON --version
 $PYTHON -m pip --version
 $PYTHON -m virtualenv -p "$PYTHON" venv
-venv/bin/python -m pip install ".[dev]"
+venv/bin/python -m pip install -e ".[dev]"
 venv/bin/python -m pip freeze
 venv/bin/python --version

.cirrus.yml

@@ -1,6 +1,6 @@
 run_tests: &RUN_TESTS
   install_cibuildwheel_script:
-    - python -m pip install ".[dev]" pytest-custom-exit-code
+    - python -m pip install -e ".[dev]" pytest-custom-exit-code
   run_cibuildwheel_tests_script:
     - python ./bin/run_tests.py
 

appveyor.yml

@@ -20,7 +20,7 @@ init:
       }
       python -m pip install -U pip
 
-install: python -m pip install ".[dev]" pytest-custom-exit-code
+install: python -m pip install -e ".[dev]" pytest-custom-exit-code
 
 # the '-u' flag is required so the output is in the correct order.
 # See https://github.com/pypa/cibuildwheel/pull/24 for more info.