urllib3 LocationParseError when installing packages with pip in remote Centos over SSH

Question

When I execute this command on a remote Centos7 host in Putty/over SSH:

pip install -r requirements.txt

I got this error

urllib3.exceptions.LocationParseError: Failed to parse: your-proxy-url:port

within following output.

Error Output

pip install -r requirements.txt
Collecting flask (from -r requirements.txt (line 1))
Exception:
Traceback (most recent call last):
  File "/var/www/centre_appel_ia/venv/lib64/python3.6/site-packages/pip/basecommand.py", line 215, in main
    status = self.run(options, args)
  File "/var/www/centre_appel_ia/venv/lib64/python3.6/site-packages/pip/commands/install.py", line 346, in run
    requirement_set.prepare_files(finder)
  File "/var/www/centre_appel_ia/venv/lib64/python3.6/site-packages/pip/req/req_set.py", line 381, in prepare_files
    ignore_dependencies=self.ignore_dependencies))
  File "/var/www/centre_appel_ia/venv/lib64/python3.6/site-packages/pip/req/req_set.py", line 557, in _prepare_file
    require_hashes
  File "/var/www/centre_appel_ia/venv/lib64/python3.6/site-packages/pip/req/req_install.py", line 278, in populate_link
    self.link = finder.find_requirement(self, upgrade)
  File "/var/www/centre_appel_ia/venv/lib64/python3.6/site-packages/pip/index.py", line 465, in find_requirement
    all_candidates = self.find_all_candidates(req.name)
  File "/var/www/centre_appel_ia/venv/lib64/python3.6/site-packages/pip/index.py", line 423, in find_all_candidates
    for page in self._get_pages(url_locations, project_name):
  File "/var/www/centre_appel_ia/venv/lib64/python3.6/site-packages/pip/index.py", line 568, in _get_pages
    page = self._get_page(location)
  File "/var/www/centre_appel_ia/venv/lib64/python3.6/site-packages/pip/index.py", line 683, in _get_page
    return HTMLPage.get_page(link, session=self.session)
  File "/var/www/centre_appel_ia/venv/lib64/python3.6/site-packages/pip/index.py", line 792, in get_page
    "Cache-Control": "max-age=600",
  File "/var/www/centre_appel_ia/venv/lib64/python3.6/site-packages/pip/_vendor/requests/sessions.py", line 543, in get
    return self.request('GET', url, **kwargs)
  File "/var/www/centre_appel_ia/venv/lib64/python3.6/site-packages/pip/download.py", line 386, in request
    return super(PipSession, self).request(method, url, *args, **kwargs)
  File "/var/www/centre_appel_ia/venv/lib64/python3.6/site-packages/pip/_vendor/requests/sessions.py", line 530, in request
    resp = self.send(prep, **send_kwargs)
  File "/var/www/centre_appel_ia/venv/lib64/python3.6/site-packages/pip/_vendor/requests/sessions.py", line 640, in send
    r = adapter.send(request, **kwargs)
  File "/var/www/centre_appel_ia/venv/lib64/python3.6/site-packages/pip/_vendor/cachecontrol/adapter.py", line 47, in send
    resp = super(CacheControlAdapter, self).send(request, **kw)
  File "/var/www/centre_appel_ia/venv/lib64/python3.6/site-packages/pip/_vendor/requests/adapters.py", line 405, in send
    conn = self.get_connection(request.url, proxies)
  File "/var/www/centre_appel_ia/venv/lib64/python3.6/site-packages/pip/_vendor/requests/adapters.py", line 303, in get_connection
    proxy_manager = self.proxy_manager_for(proxy)
  File "/var/www/centre_appel_ia/venv/lib64/python3.6/site-packages/pip/_vendor/requests/adapters.py", line 197, in proxy_manager_for
    **proxy_kwargs)
  File "/var/www/centre_appel_ia/venv/lib64/python3.6/site-packages/pip/_vendor/urllib3/poolmanager.py", line 449, in proxy_from_url
    return ProxyManager(proxy_url=url, **kw)
  File "/var/www/centre_appel_ia/venv/lib64/python3.6/site-packages/pip/_vendor/urllib3/poolmanager.py", line 394, in __init__
    proxy = parse_url(proxy_url)
  File "/var/www/centre_appel_ia/venv/lib64/python3.6/site-packages/pip/_vendor/urllib3/util/url.py", line 206, in parse_url
    raise LocationParseError(url)
pip._vendor.urllib3.exceptions.LocationParseError: Failed to parse: your-proxy-url:port

How can I solve this problem?