The following codes work by pypy 3

na=len(a)
nb=len(b)
dp=[[0]*(nb+1) for _ in range(na+1)]
for i in range(na):
    for j in range(nb):
        if a[i]==b[j]:
            dp[i+1][j+1]=dp[i][j]+1
        else:
            dp[i+1][j+1]=max(dp[i+1][j],dp[i][j+1])

suff=[[0]*(nb+1) for _ in range(na+1)]
for i in range(na-1,-1,-1):
    for j in range(nb-1,-1,-1):
        if a[i]==b[j]:
            suff[i][j]=suff[i+1][j+1]+1
        else:
            suff[i][j]=max(suff[i+1][j],suff[i][j+1])

cur = dp[na][nb]      
ret = 0
for i in range(na+1):
    used=[False]*(256)

    for j in range(nb):
        if used[ord(b[j])]==True:
            continue
        now=dp[i][j]+ suff[i][j+1] + 1
        if now == cur+1:
            used[ord(b[j])]=True
            ret+=1
return ret