Changed BASE_URL for API calls to reflect working URL and upgraded

[iamnutzy1975]

Description of change

Updated base_url variable which is used in all http requests. The previous one no longer worked.

Manual QA steps

• Use Postman to try the previous HTTP request: https://api.exchangeratesapi.io/2022-03-03?base=USD
• Use Postman to try the current (working) HTTP request: https://api.apilayer.com/exchangerates_data/2022-03-03?base=USD
  NOTE: Any historical date larger than Y2K works in the above examples

Risks

• base URL now includes a level 1 path. Not the best practice per se, but as long as every supported endpoint includes that full base URL + level 1 path, the risk is mitigated. That holds true when checking the endpoint documentation located here https://apilayer.com/marketplace/exchangerates_data-api#endpoints

Rollback steps

• revert this branch

[iamnutzy1975]

Is someone planning to review this?

[andrewelkins]

Like the update, but probably worth a few changes. Left comments.

[andrewelkins]

Remove the .idea/* files.
Keep the setup file with singer-io

[andrewelkins]

Probably worth reverting this change.