add currency pair class

tests/tests_unit/test_currency_pair.py

@@ -0,0 +1,35 @@
+from money.currency import Currency
+
+from xexe.currency_pair import CurrencyPair
+
+
+def test_create_currency_pair_normal_works_fine():
+
+    a_pair: CurrencyPair = CurrencyPair(
+        from_currency=Currency["USD"], to_currency=Currency["EUR"]
+    )
+
+    assert (a_pair.from_currency == "USD", a_pair.to_currency == "EUR")
+
+
+def test_create_currency_pair_with_same_currency_works():
+    same_curr_pair: CurrencyPair = CurrencyPair(
+        from_currency=Currency["USD"], to_currency=Currency["USD"]
+    )
+
+    assert (same_curr_pair.from_currency == "USD", same_curr_pair.to_currency == "USD")
+
+
+def test_reverse_currency_works_fine():
+
+    a_pair: CurrencyPair = CurrencyPair(
+        from_currency=Currency["USD"], to_currency=Currency["EUR"]
+    )
+
+    reverse_pair: CurrencyPair = a_pair.get_reverse_pair()
+
+    assert (
+        isinstance(reverse_pair, CurrencyPair),
+        reverse_pair.from_currency == "EUR",
+        reverse_pair.to_currency == "USD",
+    )