Suppress setup.py deprecation warnings during build (#626)

This warning is legitimate and something that developers should be aware
of, but at present, the replacement does not provide all of the
functionality necessary to support the currently available features in
colcon.

When that changes, we'll communicate the upgrade path to users and
re-enable the deprecation warning.

colcon_core/task/python/build.py

@@ -8,7 +8,6 @@
 from pathlib import Path
 import shutil
 import sys
-from sys import executable
 
 from colcon_core.environment import create_environment_hooks
 from colcon_core.environment import create_environment_scripts
@@ -26,6 +25,12 @@
 
 logger = colcon_logger.getChild(__name__)
 
+_PYTHON_CMD = [
+    sys.executable,
+    '-W',
+    'ignore:setup.py install is deprecated',
+]
+
 
 def _get_install_scripts(path):
     setup_cfg_path = os.path.join(path, 'setup.cfg')
@@ -92,7 +97,7 @@ async def build(self, *, additional_hooks=None):  # noqa: D102
 
             # invoke `setup.py install` step with lots of arguments
             # to avoid placing any files in the source space
-            cmd = [executable, 'setup.py']
+            cmd = _PYTHON_CMD + ['setup.py']
             if 'egg_info' in available_commands:
                 # `setup.py egg_info` requires the --egg-base to exist
                 os.makedirs(args.build_base, exist_ok=True)
@@ -139,8 +144,8 @@ async def build(self, *, additional_hooks=None):  # noqa: D102
             try:
                 # --editable causes this to skip creating/editing the
                 # easy-install.pth file
-                cmd = [
-                    executable, 'setup.py',
+                cmd = _PYTHON_CMD + [
+                    'setup.py',
                     'develop',
                     '--editable',
                     '--build-directory',
@@ -181,7 +186,7 @@ async def build(self, *, additional_hooks=None):  # noqa: D102
 
     async def _get_available_commands(self, path, env):
         output = await check_output(
-            [executable, 'setup.py', '--help-commands'], cwd=path, env=env)
+            _PYTHON_CMD + ['setup.py', '--help-commands'], cwd=path, env=env)
         commands = set()
         for line in output.splitlines():
             if not line.startswith(b'  '):
@@ -208,8 +213,8 @@ async def _undo_develop(self, pkg, args, env):
             args.build_base, '%s.egg-info' % pkg.name.replace('-', '_'))
         setup_py_build_space = os.path.join(args.build_base, 'setup.py')
         if os.path.exists(egg_info) and os.path.islink(setup_py_build_space):
-            cmd = [
-                executable, 'setup.py',
+            cmd = _PYTHON_CMD + [
+                'setup.py',
                 'develop',
                 '--uninstall', '--editable',
                 '--build-directory', os.path.join(args.build_base, 'build')