[WIP] cffi init_once (1.4+) support in the openssl backend

[reaperhulk]

cffi added init_once support in version 1.4. This can alleviate a race for us around the registration of the osrandom engine, but since we don't want to bump our PyPy minimum version beyond 2.6 we need to support both code paths. I am far from confident that the current PR represents the right approach, but it should serve as a starting point to figure out what we want to do.

[mention-bot]

By analyzing the blame information on this pull request, we identified @glyph, @alex and @public to be potential reviewers

[alex]

Simplifies to cffi.__version_info__ >= (1, 4, 0)

[alex]

Can we namespace these tags a bit more.

[alex]

e.g. cryptography.

[alex]

Can we do this without moving any of the code around? Ideally just altering the callsites of these functions?

[codecov-io]

Current coverage is 99.95%

Merging #2537 into master will decrease coverage by -0.03% as of c202cb3

@@            master   #2537   diff @@
======================================
  Files          123     123       
  Stmts        12960   12966     +6
  Branches      1415    1417     +2
  Methods          0       0       
======================================
+ Hit          12958   12960     +2
- Partial          2       4     +2
- Missed           0       2     +2

Review entire Coverage Diff as of c202cb3

Powered by Codecov. Updated on successful CI builds.

[alex]

Ooof, coverage is a mess here because PyPy is the one platform we don't have coverage on :-(

[reaperhulk]

The coverage issue here is potentially resolved by merging #2463, although it will significantly increase our CI time. I don't really want to put this in the twelfth release though so we can wait on this one.

[reaperhulk]

I'm unwilling to land a change like this at the moment. Mostly because until we have static callbacks sorted out I'm gunshy about making another major change. Kicking this out of the thirteenth release.