Most Python packages are using reStructuredText to format e.g. their README files. According to the //Hitchhiker’s Guide to Packaging //, README and other documentation files should have the *.txt extension.
It would be nice if Bitbucket would not only render .rst files as rst, but also .txt files. The Python Package Index does this already.
If an error occured while parsing .txt files, you could simply fall back to plain text. I don’t think this should be a problem since .rst files could also contain errors (so you should already handle them anyways).
If you are afraid that there will be a lot of errors for non-Python projects, you might activate this featere only for projects labeled as Python projects.