make the "makewheel.py" script independent from the activate_this.py script

The "activate_this.py" script only seems to be included in virtualenv,
but not venv. Now we implement its effect directly.

python/makewheel.py

@@ -1,14 +1,26 @@
 #!/usr/bin/env python
 
-import sys,os
+import sys,os,site
 
-# find python script to activate the virtual environment and source it
+base = os.path.abspath('buildwheel')
 if sys.platform == 'win32':
-  virtenv=os.path.join('buildwheel','Scripts','activate_this.py')
+  bin_dir=os.path.join(base,'Scripts')
 else:
-  virtenv=os.path.join('buildwheel','bin','activate_this.py')
+  bin_dir=os.path.join(base,'bin')
 
-exec(open(virtenv).read(), {'__file__': virtenv})
+# prepend bin to PATH, set venv path
+os.environ["PATH"] = os.pathsep.join([bin_dir] + os.environ.get("PATH", "").split(os.pathsep))
+os.environ["VIRTUAL_ENV"] = base
+
+# add the virtual environments libraries to the host python import mechanism
+prev_length = len(sys.path)
+for lib in "__LIB_FOLDERS__".split(os.pathsep):
+    path = os.path.realpath(os.path.join(bin_dir, lib))
+    site.addsitedir(path)
+sys.path[:] = sys.path[prev_length:] + sys.path[0:prev_length]
+
+sys.real_prefix = sys.prefix
+sys.prefix = base
 
 # update pip and install all requirements to build the wheel
 os.system('python -m pip install --upgrade pip')