What you want to use here is called Range
HTTP Header.
See: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html (Specifically the bit on Range).
See also API Docs on Custom Headers
Example:
from requests import get
url = "http://download.thinkbroadband.com/5MB.zip"
headers = {"Range": "bytes=0-100"} # first 100 bytes
r = get(url, headers=headers)