scripts: Fix exec invocation for in-process virtualenv activation.

activate_this.py has always documented that it should be exec()ed with
locals = globals, and in virtualenv 16.0.0 it raises a NameError
otherwise.

As a simplified demonstration of the weird things that can go wrong
when locals ≠ globals:

    >>> exec('a = 1; print([a])', {}, {})
    [1]
    >>> exec('a = 1; print([a for b in [1]])', {}, {})
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "<string>", line 1, in <module>
      File "<string>", line 1, in <listcomp>
    NameError: name 'a' is not defined
    >>> exec('a = 1; print([a for b in [1]])', {})
    [1]

Top-level assignments go into locals, but from inside a new scope like
a list comprehension, they’re read out of globals, which doesn’t work.

Fixes #12030.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

scripts/lib/setup_path_on_import.py

@@ -13,6 +13,6 @@ if sys.prefix != venv:
     # this file will exist in production
     if os.path.exists(activate_this):
         activate_locals = dict(__file__=activate_this)
-        exec(open(activate_this).read(), {}, activate_locals)
+        exec(open(activate_this).read(), activate_locals)
         if not os.path.exists(activate_locals["site_packages"]):
             raise RuntimeError(venv + " was not set up for this Python version")