Swh-graphql Code cleanup

• Add license text to source files
• Cleanup requirements
• Doc strings and comments
• Type hints
• Missing exceptions

---

Migrated from T4261 (view on Phabricator)

mentioned in commit 783a4d30

mentioned in merge request !6 (closed)

mentioned in merge request !8 (closed)

mentioned in merge request !13 (closed)

mentioned in merge request !15 (closed)

mentioned in merge request !20 (closed)

mentioned in merge request !29 (closed)

mentioned in merge request !56 (closed)

mentioned in merge request !57 (closed)

assigned to @jayeshv

marked this issue as related to #4083

added GraphQL API priority:Normal labels

mentioned in commit d5010fd7

changed the description

mentioned in commit b312a60f

mentioned in commit 267732ff

changed the description

marked this issue as related to #4306 (closed)

mentioned in commit 67f1ae16

mentioned in commit e4d4a30b