Allow passing a bytestring to XmlRfcParser

[rjsparks]

Description

Right now, we have to pass the name of a file to XmlRfcParser, which turns around and .read()s the file into memory. If we already have the content in memory, we have to do a round-trip through the filesystem that is unnecessary and adds complexity around temporary file management (see the PR at svgcheck for dealing with taking input from stdin).

Enhance the signature of init to allow passing in the bits if we have them already rather than passing a filename.

Consider deprecating passing in the filename and removing that option in the future.

Code of Conduct

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[rjsparks]

(edited to s/RfcXmlParser/XmlRfcParser/, but now I also wonder if we should start looking to change the name of the class)