Deprecate trio.ssl and trio.subprocess submodules

[oremanj]

The interesting pieces have moved to the main trio namespace; the reexports will be going away, with users accessing the standard ssl or subprocess module directly.

Closes #852.

[codecov]

Codecov Report

Merging #858 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #858      +/-   ##
==========================================
- Coverage   99.22%   99.21%   -0.02%     
==========================================
  Files         101      101              
  Lines       12182    12174       -8     
  Branches      882      880       -2     
==========================================
- Hits        12088    12078      -10     
- Misses         73       75       +2     
  Partials       21       21

Impacted Files	Coverage Δ
trio/_sync.py	99.66% <ø> (ø)	⬆️
trio/_ssl.py	98.94% <ø> (ø)	⬆️
trio/_highlevel_generic.py	100% <ø> (ø)	⬆️
trio/_deprecated_subprocess_reexports.py	100% <ø> (ø)
trio/_abc.py	100% <ø> (ø)	⬆️
trio/_deprecated_ssl_reexports.py	100% <ø> (ø)
trio/_subprocess.py	100% <ø> (ø)	⬆️
trio/tests/test_util.py	100% <100%> (ø)	⬆️
trio/tests/test_subprocess.py	100% <100%> (ø)	⬆️
trio/_highlevel_ssl_helpers.py	100% <100%> (ø)	⬆️
... and 4 more

---

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[oremanj]

Weird coverage flapping strikes again. The reported misses are in test_ssl.py, in nonsensical places like comments, blank lines, and a nonlocal statement in a function the rest of whose body is covered just fine. I get 100% coverage when I run on my local machine.

[njsmith]

Weird coverage flapping strikes again

Whatever is causing this seems to hit test_ssl.py a lot. Very weird. I guess the next step would be to dig through the coverage uploads for an individual commit and figure out where the weird lines are coming from, but that doesn't sound like a very tempting job does it. I guess we could add some flags to narrow things down... might not be a bad idea in any case.

[njsmith]

Looks good, thanks!