If you search the NCBI for information why don't you use the E-Utilities designed for this type of research? This would avoid abuse notifications returned from the site (perhaps this happened with your scraper too).
I know the question is quite old, however it can happen that somebody stumbles upon the same question...
Your base URL would be: http://eutils.ncbi.nlm.nih.gov/entrez/eutils/esearch.fcgi?db=pubmed&term=cancer+toxic+drug
You can find a description of the query parameters here: http://www.ncbi.nlm.nih.gov/books/NBK25499/#chapter4.ESearch (for more results per query and how you can advance)
And using this API would you enable you to use some other tools and a newer Python 3 too.