Support loading EC keys from pem in KeyBundle

[angelakis]

Related to Issue #34.

I went a bit out of my way to fix keybundle_from_local_file to work for EC too and add P-256 as a default curve in ec_init (like rsa_init has 2048 as a default size). I also cleaned up some related tests.

[codecov]

Codecov Report

Merging #35 into master will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
+ Coverage   77.48%   77.55%   +0.06%     
==========================================
  Files          37       37              
  Lines        3682     3693      +11     
  Branches      713      715       +2     
==========================================
+ Hits         2853     2864      +11     
  Misses        603      603              
  Partials      226      226              

Impacted Files	Coverage Δ
src/cryptojwt/key_bundle.py	81.63% <0.00%> (+0.38%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 45b6878...4da5162. Read the comment docs.