安装pip3出错：
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
libpython3-dev libpython3.5-dev python3-dev python3-setuptools python3-wheel python3.5-dev
Suggested packages:
python-setuptools-doc
The following NEW packages will be installed:
libpython3-dev libpython3.5-dev python3-dev python3-pip python3-setuptools python3-wheel python3.5-dev
0 upgraded, 7 newly installed, 0 to remove and 0 not upgraded.
Need to get 37.7 MB/38.0 MB of archives.
After this operation, 55.2 MB of additional disk space will be used.
Do you want to continue? [Y/n] y
Ign:1 http://mirrors.cloud.aliyuncs.com/ubuntu xenial-security/main amd64 libpython3.5-dev amd64 3.5.2-2ubuntu0~16.04.1
Ign:2 http://mirrors.cloud.aliyuncs.com/ubuntu xenial-security/main amd64 python3.5-dev amd64 3.5.2-2ubuntu0~16.04.1
Err:1 http://mirrors.cloud.aliyuncs.com/ubuntu xenial-security/main amd64 libpython3.5-dev amd64 3.5.2-2ubuntu0~16.04.1
404 Not Found
Err:2 http://mirrors.cloud.aliyuncs.com/ubuntu xenial-security/main amd64 python3.5-dev amd64 3.5.2-2ubuntu0~16.04.1
404 Not Found
E: Failed to fetch http://mirrors.cloud.aliyuncs.com/ubuntu/pool/main/p/python3.5/libpython3.5-dev_3.5.2-2ubuntu0~16.04.1_amd64.deb 404 Not Found
E: Failed to fetch http://mirrors.cloud.aliyuncs.com/ubuntu/pool/main/p/python3.5/python3.5-dev_3.5.2-2ubuntu0~16.04.1_amd64.deb 404 Not Found
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?执行一下apt-get update更新源