venv: Add a command-line package for creating virtual environments.

Works like "python -m venv path" and creates a rudimentary virtual
environment for the Unix port:
 - sets MICROPYPATH
 - copies the micropython binary to venv/bin/micropython which is in $PATH
 - installs mip & mip-cmdline in the venv

Using the venv is the same as for CPython -- source the activate script to
enter, and call the deactivate function to leave.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

python-stdlib/venv/manifest.py

@@ -0,0 +1,9 @@
+metadata(
+    version="0.1.0",
+    description="Support for creating MicroPython virtual environments using `micropython -m venv`",
+)
+
+require("argparse")
+require("mip-cmdline")
+
+package("venv")

python-stdlib/venv/venv/__main__.py

@@ -0,0 +1,99 @@
+# Support for creating MicroPython virtual environments using `micropython -m venv`
+# MIT license; Copyright (c) 2022 Jim Mussared
+
+import argparse
+import os
+import sys
+
+
+# If mip is not frozen into this binary, then also install it in the venv.
+def install_mip(venv_lib_path):
+    need_mip = False
+    if "mip" in sys.modules:
+        del sys.modules["mip"]
+    saved_sys_path = sys.path[:]
+    try:
+        sys.path[:] = [".frozen"]
+        try:
+            import mip
+
+            print("mip is frozen")
+        except ImportError:
+            need_mip = True
+    finally:
+        sys.path[:] = saved_sys_path
+
+    if need_mip:
+        import mip
+
+        mip.install("mip-cmdline", target=venv_lib_path)
+
+
+def do_venv():
+    parser = argparse.ArgumentParser(description="Create a micropython virtual environment")
+    parser.add_argument("path", nargs=1, help="Path to create the virtual environment in")
+    args = parser.parse_args(args=sys.argv[1:])
+    venv_path = args.path[0]
+    print("Creating virtual environment in:", venv_path)
+
+    # Equivalent to path = os.abspath(path).
+    if not venv_path.startswith("/"):
+        venv_path = os.getcwd() + os.sep + venv_path
+
+    venv_bin_path = venv_path + os.sep + "bin"
+    venv_lib_path = venv_path + os.sep + "lib"
+
+    for d in (
+        venv_path,
+        venv_bin_path,
+        venv_lib_path,
+    ):
+        try:
+            os.mkdir(d)
+        except:
+            pass
+
+    # Note the venv/lib dir goes before .frozen so that installed packages replace frozen ones.
+    with open(venv_bin_path + os.sep + "activate", "w") as f:
+        print(
+            """# Usage: source bin/activate
+
+deactivate() {{
+    PATH="$_OLD_VIRTUAL_PATH"
+    export PATH
+
+    MICROPYPATH="$_OLD_VIRTUAL_MICROPYPATH"
+    if [ -z "$MICROPYPATH" ]; then
+        export -n MICROPYPATH
+    else
+        export MICROPYPATH
+    fi
+
+    unset VIRTUAL_ENV
+
+    unset deactivate
+}}
+
+VIRTUAL_ENV={}
+
+_OLD_VIRTUAL_PATH="$PATH"
+PATH="$VIRTUAL_ENV/bin:$PATH"
+export PATH
+
+_OLD_VIRTUAL_MICROPYPATH="$MICROPYPATH"
+MICROPYPATH="$VIRTUAL_ENV/lib:.frozen"
+export MICROPYPATH
+""".format(
+                venv_path
+            ),
+            file=f,
+        )
+
+    # Add a `micropython` binary in $PATH pointing to this binary.
+    if hasattr(sys, "executable"):
+        os.system("cp {} {}".format(sys.executable, venv_bin_path + os.sep + "micropython"))
+
+    install_mip(venv_lib_path)
+
+
+do_venv()