Replace forex-python and fetch rates directly from EU website

https://theforexapi.com/ is frequently down which breaks the tests. The EU
website is more reliable.
As forex-python is just also used to set the symbol of currencies when
importing, we can just fill the symbol when it is empty with the alpha-3 code.

issue11731
review439631003
(grafted from 2097ae9311186817cd70c172c478d102435ad2bf)

--HG--
branch : 6.0

scripts/import_currencies.py

@@ -7,7 +7,6 @@ import sys
 from argparse import ArgumentParser
 
 import pycountry
-from forex_python.converter import CurrencyCodes
 
 try:
     from progressbar import ProgressBar, Bar, ETA, SimpleProgress
@@ -53,7 +52,6 @@ def get_currencies():
 def update_currencies(currencies):
     print("Update currencies", file=sys.stderr)
     Currency = Model.get('currency.currency')
-    codes = CurrencyCodes()
 
     records = []
     for currency in _progress(pycountry.currencies):
@@ -61,10 +59,11 @@ def update_currencies(currencies):
         if code in currencies:
             record = currencies[code]
         else:
-            record = Currency(code=code)
+            record = Currency(code=code, symbol=None)
         record.name = _remove_forbidden_chars(currency.name)
         record.numeric_code = currency.numeric
-        record.symbol = codes.get_symbol(currency.alpha_3) or currency.alpha_3
+        if not record.symbol:
+            record.symbol = currency.alpha_3
         records.append(record)
 
     Currency.save(records)