参考网站：
https://stackoverflow.com/questions/510357/python-read-a-single-character-from-the-user
https://stackoverflow.com/questions/2988211/how-to-read-a-single-character-at-a-time-from-a-file-in-python
从stdin中逐字符读入：
sys.stdin.read(1)从文件中逐字符读入：
f = open('test.txt')
a = f.read(1)例子：
echo程序
从stdin读入：
import sys
if __name__ == '__main__':
a = sys.stdin.read(1)
while a:
print(a, end="")
a = sys.stdin.read(1)从test.txt读入：
import sys
if __name__ == '__main__':
f = open('test.txt')
a = f.read(1)
while a:
print(a, end="")
a = f.read(1)