patch Popen.__init__, not Popen

asyncio.windows_utils subclasses/aliases Popen like this:

```python
class Popen(subprocess.Popen)
  def __init__(...):
    ...
    try:
      super().__init__(...)
    ...

```

Calling `partial()` on Popen directly works when
using it like a function, but breaks when
subclassing Popen (like asyncio does).

Closes rstudio/keras#1375
Closes #1478

inst/python/rpytools/subprocess.py

@@ -6,4 +6,4 @@ def patch_subprocess_Popen():
   import subprocess
   from functools import partial
 
-  subprocess.Popen = partial(subprocess.Popen, stdin = subprocess.DEVNULL)
+  subprocess.Popen.__init__ = partial(subprocess.Popen.__init__, stdin = subprocess.DEVNULL)