Don't catch the SerialException we just raised

[kalvdans]

In Python3 we get double tracebacks for this error:

Traceback (most recent call last):
  File "/home/chn/repo/pyserial/serial/serialposix.py", line 557, in read
    raise SerialException(
serial.serialutil.SerialException: device reports readiness to read but returned no data (device disconnected or multiple access on port?)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "test/test_exceptions.py", line 26, in <module>
    test_unexpected_eof()
  File "/usr/lib/python3.8/unittest/mock.py", line 1348, in patched
    return func(*newargs, **newkeywargs)
  File "test/test_exceptions.py", line 21, in test_unexpected_eof
    s.read()
  File "/home/chn/repo/pyserial/serial/serialposix.py", line 566, in read
    raise SerialException('read failed: {}'.format(e))
serial.serialutil.SerialException: read failed: device reports readiness to read but returned no data (device disconnected or multiple access on port?)

The patch moves the checking for EOF to after the IO block, resulting
in a much nicer traceback. This is the test script (Python3-specific):

from unittest import mock
import serial
import select
import os

@mock.patch('select.select')
@mock.patch('os.read')
def test_unexpected_eof(mock_read, mock_select):
    s = serial.Serial()
    s.is_open = True
    s.fd = 99
    s.pipe_abort_read_r = 98
    mock_select.return_value = ([99],[],[])
    mock_read.return_value = b''
    try:
        s.read()
    except serial.SerialException as e:
        if e.__context__: raise

if __name__ == '__main__':
    test_unexpected_eof()

[zsquareplusc]

Makes sense. Thank you