Don't add not supported SSL_ST_* into _all_, fixies #738

[onovy]

Without this patch this fails:

from OpenSSL.SSL import *
Traceback (most recent call last):
File "", line 1, in
File "/usr/lib/python2.7/dist-packages/cryptography/utils.py", line 124, in getattr
obj = getattr(self._module, attr)
AttributeError: 'module' object has no attribute 'SSL_ST_INIT'

[codecov]

Codecov Report

Merging #739 into master will decrease coverage by 2.78%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #739      +/-   ##
==========================================
- Coverage   97.05%   94.27%   -2.79%     
==========================================
  Files          18       17       -1     
  Lines        5711     5711              
  Branches      395      391       -4     
==========================================
- Hits         5543     5384     -159     
- Misses        112      262     +150     
- Partials       56       65       +9

Impacted Files	Coverage Δ
src/OpenSSL/SSL.py	89.8% <100%> (-5.15%)	⬇️
src/OpenSSL/_util.py	81.13% <0%> (-9.44%)	⬇️
tests/test_ssl.py	93.76% <0%> (-5.36%)	⬇️
tests/util.py	93.44% <0%> (-3.28%)	⬇️
src/OpenSSL/crypto.py	96.25% <0%> (-0.61%)	⬇️
tests/__init__.py

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