fix: Add missing DOIs to paper.bib (JOSS bot feedback)

Author: rahuldass19

paper.bib

@@ -4,7 +4,8 @@ @article{demoura2008z3
   journal={Tools and Algorithms for the Construction and Analysis of Systems},
   pages={337--340},
   year={2008},
-  publisher={Springer}
+  publisher={Springer},
+  doi={10.1007/978-3-540-78800-3_24}
 }
 
 @article{meurer2017sympy,
@@ -13,7 +14,8 @@ @article{meurer2017sympy
   journal={PeerJ Computer Science},
   volume={3},
   pages={e103},
-  year={2017}
+  year={2017},
+  doi={10.7717/peerj-cs.103}
 }
 
 @misc{sqlglot,