parser: Use updated python-asn1 API

[m1stadev]

python-asn1 2.7.0 adds context manager support for asn1.Encoder. PyIMG4's parsing code can be updated to utilize this.

There is also a version by @dhinakg here that implements context manager support for asn1.Decoder, which could also be included in PyIMG4's source (though I'd rather a fork is made first).

[m1stadev]

PyIMG4 has moved to utilizing python-asn1's context manager for asn1.Encoder in 63f92d2.