系统为Centos7.8，默认已经安装了Python2.7.5。
Python3和Python2可以共存，安装Python3不要卸载Python2，因为yum依赖于Python2。

刚开始不清楚，按照网上教程删除了Python2和yum。
1、删除现有Python

rpm -qa|grep python|xargs rpm -ev --allmatches --nodeps
whereis python |xargs rm -frv
whereis python

2、删除现有的yum

rpm -qa|grep yum|xargs rpm -ev --allmatches --nodeps
whereis yum |xargs rm -frv

结果发现后面安装Python3需要yum，于是只能重装Python2和yum

1、查看本机CentOS版本

cat /etc/redhat-release

结果：CentOS Linux release 7.8.2003 (Core)

2、创建python和yum两个文件夹

mkdir /usr/local/src/python
mkdir /usr/local/src/yum

3、根据上面版本7.8.2003

访问：Index of /7.8.2003/os/x86_64/Packages
下载下面文件，放到/usr/local/src/python：

dbus-python-devel-1.1.1-9.el7.x86_64.rpm
python-2.7.5-88.el7.x86_64.rpm
python-iniparse-0.4-9.el7.noarch.rpm
python-libs-2.7.5-88.el7.x86_64.rpm
python-pycurl-7.19.0-19.el7.x86_64.rpm
python-urlgrabber-3.10-10.el7.noarch.rpm
rpm-python-4.11.3-43.el7.x86_64.rpm

下载下面文件，放到/usr/local/src/yum

yum-3.4.3-167.el7.centos.noarch.rpm
yum-metadata-parser-1.1.4-10.el7.x86_64.rpm
yum-plugin-fastestmirror-1.1.31-53.el7.noarch.rpm

4、安装python

cd /usr/local/src/python
rpm -ivh python-* rpm-python-*

如果出现安装包依赖问题，执行

rpm -ivh python-* rpm-python-* --nodeps --force

5、安装yum

cd /usr/local/src/yum
rpm -ivh yum-*

安装Python3.7

1.安装编译工具和依赖环境等

yum -y groupinstall "Development tools"
yum -y install zlib-devel bzip2-devel openssl-devel ncurses-devel sqlite-devel readline-devel tk-devel gdbm-devel db4-devel libpcap-devel xz-devel
yum install libffi-devel -y

2.下载Python3安装包、解压

wget https://www.python.org/ftp/python/3.7.0/Python-3.7.0.tar.xz
tar -zxvf Python-3.7.0.tar.xz

3.编译安装

mkdir /usr/local/python3
cd Python-3.7.0
./configure --prefix=/usr/local/python3
make && make install

4.建立软连接

ln -s /usr/local/python3/bin/python3 /usr/local/bin/python3
ln -s /usr/local/python3/bin/pip3 /usr/local/bin/pip3

5.验证是否成功

python3 -V
pip3 -V