给定一个字符串，计算其中每个字符出现的频率。
字符串：“Python is a widely used high-level programming language for general-purpose programming, created by Guido van Rossum and first released in 1991.”
P: 2
y: 2
t: 7
h: 5
o: 7
n: 9
 : 23
i: 6
s: 5
a: 7
w: 1
d: 6
e: 12
l: 5
u: 3
v: 1
p: 3
r: 7
g: 7
m: 3
-: 1
c: 3
,: 2
f: 3
b: 1
1991: 1.