feat(venv): add trame.env.venv

This works identically to the paraview web venv. You specify either the arguments "--venv <path>"
or the "TRAME_VENV" environment variable. Then, if `trame.env.venv` is imported, that virtual
environment path will be used instead of the original one.

Signed-off-by: Patrick Avery <patrick.avery@kitware.com>

trame/env/venv.py

@@ -0,0 +1,36 @@
+# -*- coding: utf-8 -*-
+"""Activate venv for current interpreter:
+
+Use `import trame.env.venv` along one of the following
+ - `--venv /path/to/venv/base` argument
+ - environment variable `TRAME_VENV=/path/to/venv/base`
+
+This can be used when you must use an existing Python interpreter, not the venv bin/python.
+"""
+import os
+import site
+import sys
+
+VENV_BASE = None
+VENV_LOADED = False
+
+if os.environ.get("TRAME_VENV"):
+    VENV_BASE = os.path.abspath(os.environ.get("TRAME_VENV"))
+
+if "--venv" in sys.argv:
+    VENV_BASE = os.path.abspath(sys.argv[sys.argv.index("--venv") + 1])
+
+if not VENV_LOADED and VENV_BASE and os.path.exists(VENV_BASE):
+    VENV_LOADED = True
+    # Code inspired by virutal-env::bin/active_this.py
+    bin_dir = os.path.join(VENV_BASE, "bin")
+    os.environ["PATH"] = os.pathsep.join([bin_dir] + os.environ.get("PATH", "").split(os.pathsep))
+    os.environ["VIRTUAL_ENV"] = VENV_BASE
+    prev_length = len(sys.path)
+    python_libs = os.path.join(VENV_BASE, f"lib/python{sys.version_info.major}.{sys.version_info.minor}/site-packages")
+    site.addsitedir(python_libs)
+    sys.path[:] = sys.path[prev_length:] + sys.path[0:prev_length]
+    sys.real_prefix = sys.prefix
+    sys.prefix = VENV_BASE
+    #
+    print(f"Trame is using venv: {VENV_BASE}")