Declare ecdsa as the module Python dependency

The bundled BTCPay client (models/libs) imports ecdsa, while the manifest
and requirements only listed btcpay-python, which is not used (the client
is vendored). Installations that did not already have ecdsa available
failed the external-dependency check. Declare ecdsa instead and bump the
module version.

payment_btcpayserver/requirements.txt

@@ -1 +1 @@
-btcpay-python
+ecdsa