Unclosed files

[tpazderka]

The code leaves behind some unclosed files. This is a resource waste and produces warnings under Python 3.

Some examples in code:
https://github.com/IdentityPython/pyjwkest/blob/master/src/jwkest/jwk.py#L93
https://github.com/IdentityPython/pyjwkest/blob/master/src/jwkest/jwk.py#L164

And in tests:
https://github.com/IdentityPython/pyjwkest/blob/master/tests/test_2_jwk.py#L173
https://github.com/IdentityPython/pyjwkest/blob/master/tests/test_2_jwk.py#L297
https://github.com/IdentityPython/pyjwkest/blob/master/tests/test_3_jws.py#L632