Be able to configure the underlying connection pool size

[gael-ft]

Hello,

Would be nice to be able to configure the underlying size of the connection pool.
DeepL is using requests as:

self._session = requests.Session()

which uses the default HTTPAdater, so 10 connections for http:// & https://.

When we have a peak, underlying client logs things like Connection pool is full, discarding connection which is not an error as is, but would be better if we could adapt the pool according to our traffic.