linux运行pip时报错解决

linux下 安装Pip后，使用Pip安装包时报错：

-bash-4.1$ pip install requests --trusted-host mirrors.aliyun.com
WARNING: pip is configured with locations that require TLS/SSL, however the ssl module in Python is not available.
Collecting requests
  WARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError("Can't connect to HTTPS URL because the SSL module is not available.",)': /simple/requests/
  WARNING: Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError("Can't connect to HTTPS URL because the SSL module is not available.",)': /simple/requests/
  WARNING: Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError("Can't connect to HTTPS URL because the SSL module is not available.",)': /simple/requests/
  WARNING: Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError("Can't connect to HTTPS URL because the SSL module is not available.",)': /simple/requests/
  WARNING: Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError("Can't connect to HTTPS URL because the SSL module is not available.",)': /simple/requests/
  Could not fetch URL  There was a problem confirming the ssl certificate: HTTPSConnectionPool(host='pypi.org', port=443): Max retries exceeded with url: /simple/requests/ (Caused by SSLError("Can't connect to HTTPS URL because the SSL module is not available.",)) - skipping
  ERROR: Could not find a version that satisfies the requirement requests (from versions: none)
ERROR: No matching distribution found for requests
WARNING: pip is configured with locations that require TLS/SSL, however the ssl module in Python is not available.
Could not fetch URL  There was a problem confirming the ssl certificate: HTTPSConnectionPool(host='pypi.org', port=443): Max retries exceeded with url: /simple/pip/ (Caused by SSLError("Can't connect to HTTPS URL because the SSL module is not available.

用过各种方法都不行，添加参数后成功：

pip install pymysql --trusted-host mirrors.aliyun.com --index-url http://mirrors.aliyun.com/pypi/simple/