Migrate to API v2 and add OHLC endpoint

[oligirling]

Summary

• Breaking: Migrate all requests from API v1 to v2 (https://currencyapi.net/api/v2/)
• Add new ohlc() endpoint with currency, date, base, and interval parameters (Tier 3+)
• Bump version to 2.0.0

Changes

• endpoint.py — API_VERSION updated from v1 to v2
• ohlc.py — New OHLC endpoint class with fluent interface
• currency.py — Added ohlc() factory method
• tests/endpoints/test_ohlc.py — Full test suite for OHLC endpoint
• tests/test_currency.py — Added test_ohlc_method
• README.md — OHLC endpoint docs with parameter table, examples, and sample response
• CHANGELOG.md — [2.0.0] entry documenting breaking changes and new endpoint
• setup.py — Version bumped to 2.0.0

Test plan

• All 45 tests pass in Docker (python -m coverage run -m unittest discover)
• 100% test coverage maintained across all modules

🤖 Generated with Claude Code

[coveralls]

Pull Request Test Coverage Report for Build 22175049469

Details

• 71 of 71 (100.0%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

---

Totals
Change from base Build 21981060819:	0.0%
Covered Lines:	380
Relevant Lines:	380

---

💛 - Coveralls