ECDSA Signing and verifying signatures between Python and JS
An ECDSA signature is, formally, a pair of integers $(r,s)$. There are two main conventions for encoding these integers into bytes:


Encode both integers into unsigned big-endian, using the same size for both, and concatenate the values. This is the traditional way in, for instance, PKCS#11 and OpenPGP; python-ecdsa apparently uses that format.
Encode the integers as an ASN.1/DER structure (a SEQUENCE of two INTEGER values). This is what is normally used in everything that relates to X.509 certificates, and also in SSL/TLS exchanges. jsrsasign apparently uses that format.


Conversion between these formats can be done, but it is surprisingly tricky to do correctly (it's a parser, after all). It seems that python-ecdsa can also encode and decode ASN.1-based signatures (see the functions sigencode_der() and sigdecode_der(), for instance).