xopen 2.0.0 does not read stdin fully

Originally logged as https://github.com/marcelm/cutadapt/issues/774 but reduced to an xopen only test case by @rhpvorderman : ``` (xopen) rhpvorderman@tuxminator:~/PycharmProjects/xopen$ pip list | grep xopen xopen 2.0.0 (xopen) rhpvorderman@tuxminator:~/PycharmProjects/xopen$ wc -l ~/test/5millionreads_R1.fastq 20000000 /home/rhpvorderman/test/5millionreads_R1.fastq (xopen) rhpvorderman@tuxminator:~/PycharmProjects/xopen$ cat ~/test/5millionreads_R1.fastq | python -c 'import xopen; f=xopen.xopen("/dev/stdin", "rt"); print(f.read())' | wc -l 19999956 (xopen) rhpvorderman@tuxminator:~/PycharmProjects/xopen$ cat ~/test/5millionreads_R1.fastq | python -c 'import xopen; f=xopen.xopen("-", "rt"); print(f.read())' | wc -l 19999956 (xopen) rhpvorderman@tuxminator:~/PycharmProjects/xopen$ pip install xopen==1.9.0 >/dev/null (xopen) rhpvorderman@tuxminator:~/PycharmProjects/xopen$ cat ~/test/5millionreads_R1.fastq | python -c 'import xopen; f=xopen.xopen("-", "rt"); print(f.read())' | wc -l 20000001 (xopen) rhpvorderman@tuxminator:~/PycharmProjects/xopen$ cat ~/test/5millionreads_R1.fastq | python -c 'import xopen; f=xopen.xopen("/dev/stdin", "rt"); print(f.read())' | wc -l 20000001 (xopen) rhpvorderman@tuxminator:~/PycharmProjects/xopen$ ``` The +1 is from print adding a new line.