Fix CITATION file so it specifies pytest as software.

[willynilly]

The CITATION file is using BibTex format that lacks metadata for citing software. It needs to include an example of BibLaTex file which has additional software specific metadata. I had tried importing the BibTex into a citation manager like Zotero, and it indicated that the citation was a document. However, using BibLaTex will indicate that it is of the type software. This facilitates citation. Also, the authors section of the example is incorrectly formatted. The author names should be "{firstname} {lastname}" and not "{lastname}, {firstname}". The version information should also be included in a note for the BibTex format. Finally, it needs an example of how to create a BibLaTex format in the CITATION.

I corrected the issue and will submit a pull request for it shortly.

• [ X] a detailed description of the bug or problem you are having
• [ N/A] output of pip list from the virtual environment you are using
• [ N/A] pytest and operating system versions
• [ X] minimal example if possible