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 was copied to tuf/scripts/ to "make testing
easier" (cf84d3f), although with
the current test setup the original (and recently patched to fix an
Windows/Py2 test issue) test simple_server.py can be used just as
well.

This commit:
- removes tuf/scripts/simple_server.py
  Note: that version slightly differed from the original test
  server, probably due to demands by the linter that is only executed
  on the tuf core code and not on the tests. However, for the testing
  purposes of simple_server.py these changes (i.e., `SystemRandom()`,
  `if __name__ =='__main__':`) are not necessary.
- updates the tests that used tuf.scripts.simple_server to instead
  use tests.simple_server,
- updates setup.py to not install the simple_server module as
  script, when installing tuf, as it is only a testing script and
  not meant for end-user usage.

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