Adjust to HMRC moving older exchanges rates.

[nihn]

Data for years prior to 2021 was moved to a different service.

Trying to run without the changes:

mateuszm@non-client-XR974FX65W capital-gains-calculator-forked % /Users/mateuszm/.local/bin/poetry run cgt-calc  --raw ../capital-gains-calculator/test2.csv --no-balance-check --no-pdflatex --year 2022
INFO: No schwab file provided
INFO: No schwab Equity Award JSON file provided
INFO: No trading212 folder provided
INFO: No mssb folder provided
INFO: No sharesight file provided
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/mateuszm/github/capital-gains-calculator-forked/cgt_calc/main.py", line 713, in init
    sys.exit(main())
             ^^^^^^
  File "/Users/mateuszm/github/capital-gains-calculator-forked/cgt_calc/main.py", line 692, in main
    acquisition_list, disposal_list = calculator.convert_to_hmrc_transactions(
                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/mateuszm/github/capital-gains-calculator-forked/cgt_calc/main.py", line 198, in convert_to_hmrc_transactions
    self.add_acquisition(portfolio, acquisition_list, transaction)
  File "/Users/mateuszm/github/capital-gains-calculator-forked/cgt_calc/main.py", line 128, in add_acquisition
    self.converter.to_gbp_for(amount, transaction),
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/mateuszm/github/capital-gains-calculator-forked/cgt_calc/currency_converter.py", line 115, in to_gbp_for
    return self.to_gbp(amount, transaction.currency, transaction.date)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/mateuszm/github/capital-gains-calculator-forked/cgt_calc/currency_converter.py", line 110, in to_gbp
    return amount / self.currency_to_gbp_rate(currency.upper(), date)
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/mateuszm/github/capital-gains-calculator-forked/cgt_calc/currency_converter.py", line 101, in currency_to_gbp_rate
    self._query_hmrc_api(month_str)
  File "/Users/mateuszm/github/capital-gains-calculator-forked/cgt_calc/currency_converter.py", line 81, in _query_hmrc_api
    raise ParsingError(
cgt_calc.exceptions.ParsingError: While parsing http://www.hmrc.gov.uk/softwaredevelopers/rates/exrates-monthly-1123.xml, HMRC API returned a 504 response

After:

mateuszm@non-client-XR974FX65W capital-gains-calculator-forked % /Users/mateuszm/.local/bin/poetry run cgt-calc  --raw ../capital-gains-calculator/test2.csv --no-balance-check --no-pdflatex --year 2022
INFO: No schwab file provided
INFO: No schwab Equity Award JSON file provided
INFO: No trading212 folder provided
INFO: No mssb folder provided
INFO: No sharesight file provided
First pass completed
Final portfolio:
...

Veryfied that caching of the exchange rates works as well (ran twice):

mateuszm@non-client-XR974FX65W capital-gains-calculator-forked % /Users/mateuszm/.local/bin/poetry run cgt-calc  --raw ../capital-gains-calculator/test2.csv --no-balance-check --no-pdflatex --year 2022 --exchange-rates-file rates.csv
INFO: No schwab file provided
INFO: No schwab Equity Award JSON file provided
INFO: No trading212 folder provided
INFO: No mssb folder provided
INFO: No sharesight file provided
First pass completed
...

Also this diff starts using requests.Session to speed up things a bit.

[KapJI]

Thanks a lot!