Description of problem:
When using the KEMClient from the custodia.message.kem, it's impossible to get KEMClient.parse_reply() working since its second positional argument throws exception when either bytes or string instance is passed to it.


Version-Release number of selected component (if applicable):
custodia-0.5.0-1.fc26.noarch

How reproducible:
Always

Steps to Reproduce:
1. Have your own KEMClient set up and get yourself a reply from custodia
2a. Try to parse the reply using KEMClient.parse_reply()
2b. Try to parse the raw reply (bytes) using KEMClient.parse_reply()

Actual results:
str passed as the second argument:
```  File "/usr/lib/python3.6/site-packages/custodia/message/kem.py", line 221, in parse_reply
    self.server_keys[KEY_USAGE_SIG])
  File "/usr/lib/python3.6/site-packages/custodia/message/kem.py", line 245, in decode_enc_kem
    jwe = JWT(jwt=message, key=enc_key)
  File "/usr/lib/python3.6/site-packages/jwcrypto/jwt.py", line 204, in __init__
    self.deserialize(jwt, key)
  File "/usr/lib/python3.6/site-packages/jwcrypto/jwt.py", line 460, in deserialize
    self.token.deserialize(jwt, key)
  File "/usr/lib/python3.6/site-packages/jwcrypto/jwe.py", line 1239, in deserialize
    raise InvalidJWEData('Invalid format', repr(e))
```


bytes passed:
```
  File "/usr/lib/python3.6/site-packages/custodia/message/kem.py", line 221, in parse_reply
    self.server_keys[KEY_USAGE_SIG])
  File "/usr/lib/python3.6/site-packages/custodia/message/kem.py", line 245, in decode_enc_kem
    jwe = JWT(jwt=message, key=enc_key)
  File "/usr/lib/python3.6/site-packages/jwcrypto/jwt.py", line 204, in __init__
    self.deserialize(jwt, key)
  File "/usr/lib/python3.6/site-packages/jwcrypto/jwt.py", line 443, in deserialize
    c = jwt.count('.')
```
Expected results: 
The string or bytes or whatever needs to be passed to the parse_reply() method get properly parsed.

Additional info:
This happens during FreeIPA ipa-replica-install on domain level 1 which for me is the easiest way to reproduce this. Contact me for a patched version of FreeIPA which would allow you getting to the spot if you need.

Note that this is BLOCKING FreeIPA Python 3 adoption in Fedora 27, thus the severity.