Fix failing AppVeyor Python2.7 tests

Since theupdateframework#885 the tests in TestUpdater and TestKeyRevocation fail on
Appveyor Python 2.7 builds. After some live debugging, it turns out
that the tests fail due to the extra amount of http requests to
the simple http server (see tests/simple_server.py) that were
added in theupdateframework#885.

The simple server runs in a subprocess and is re-used for the
entire TestCase. After a certain amount of requests it becomes
unresponsive. Note that neither the subprocess exits (ps -W), nor
does the port get closed (netstat -a). It just doesn't serve the
request, making it time out and fail the test.

The following script can be used to reproduce the issue (run in
tests directory):

```python
import subprocess
import requests
import random

counter = 0

port = random.randint(30000, 45000)
command = ['python', 'simple_server.py', str(port)]
server_process = subprocess.Popen(command, stderr=subprocess.PIPE)
url = 'http://localhost:'+str(port) + '/'

sess = requests.Session()

try:
  while True:
    sess.get(url, timeout=3)
    counter +=1

finally:
  print(counter)
  server_process.kill()
```

It fails repeatedly on the 69th request, but only if
`stderr=subprocess.PIPE` is passed to Popen. Given that for each
request the simple server writes about ~60 characters to stderr,
e.g. ...
```
127.0.0.1 - - [24/Feb/2020 12:01:23] "GET / HTTP/1.1" 200 -
```
... it looks a lot like a full pipe buffer of size 4096. Note that the
`bufsize` argument to Popen does not change anything.

As a simple work around we silence the test server on
Windows/Python2 to not fill the buffer.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

tests/simple_server.py

@@ -37,6 +37,7 @@
 import random
 
 import six
+from six.moves.SimpleHTTPServer import SimpleHTTPRequestHandler
 
 PORT = 0
 
@@ -55,7 +56,22 @@ def _port_gen():
 else:
   PORT = _port_gen()
 
-Handler = six.moves.SimpleHTTPServer.SimpleHTTPRequestHandler
-httpd = six.moves.socketserver.TCPServer(('', PORT), Handler)
+
+class QuietHTTPRequestHandler(SimpleHTTPRequestHandler):
+  """A SimpleHTTPRequestHandler that does not write incoming requests to
+  stderr. """
+  def log_request(self, code='-', size='-'):
+    pass
+
+# NOTE: On Windows/Python2 tests that use this simple_server.py in a
+# subprocesses hang after a certain amount of requests (~68), if a PIPE is
+# passed as Popen's stderr argument. As a simple workaround we silence the
+# server on those Windows/Py2 to not fill the buffer.
+if six.PY2 and platform.system() == 'Windows':
+  handler = QuietHTTPRequestHandler
+else:
+  handler = SimpleHTTPRequestHandler
+
+httpd = six.moves.socketserver.TCPServer(('', PORT), handler)
 
 httpd.serve_forever()