Add types to _signals, test_signals

[TeamSpen210]

This is fairly straightforward, but I did extract a test helper method into a function. This allows open_signal_receiver() to be typed as producing an AsyncIterator, keeping SignalReceiver internal.

[codecov]

Codecov Report

Merging #2794 (7f27398) into master (be39867) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2794   +/-   ##
=======================================
  Coverage   98.95%   98.95%           
=======================================
  Files         115      115           
  Lines       17003    17011    +8     
  Branches     3045     3045           
=======================================
+ Hits        16825    16833    +8     
  Misses        123      123           
  Partials       55       55           

Files Changed	Coverage Δ
trio/_signals.py	100.00% <100.00%> (ø)
trio/_tests/test_signals.py	100.00% <100.00%> (ø)

[CoolCat467]

Looks good to me. If anything, the change removing _pending_signal_count could potentially cause issues if people are using this function even though it basically says not to because it was only there for the tests, but I think this is an acceptable change because:

1. People shouldn't be using said function anyways
2. Making it a separate function that isn't publicly exposed is a better solution for the tests than having it be a class function that technically could exposed

[A5rocks]

Looks like a pretty simple file to type. Essentially no comments!

[A5rocks]

Looks good to me.