Add a variety of type annotations for mypy

[william-stearns]

Replacing #597 (I'm sure there's a more elegant way to do this.) Fixes a single missing "import io" in stream_interface.py

[codecov]

Codecov Report

Attention: Patch coverage is 85.10638% with 21 lines in your changes missing coverage. Please review.

Project coverage is 60.35%. Comparing base (0deb1d7) to head (384063d).
Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
meshtastic/ble_interface.py	43.75%	9 Missing ⚠️
meshtastic/stream_interface.py	70.00%	6 Missing ⚠️
meshtastic/util.py	94.00%	3 Missing ⚠️
meshtastic/__main__.py	90.00%	2 Missing ⚠️
meshtastic/tcp_interface.py	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #681      +/-   ##
==========================================
- Coverage   60.37%   60.35%   -0.03%     
==========================================
  Files          24       24              
  Lines        3793     3811      +18     
==========================================
+ Hits         2290     2300      +10     
- Misses       1503     1511       +8     

Flag	Coverage Δ
unittests	60.35% <85.10%> (-0.03%)	⬇️

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[ianmcorvidae]

Hi @william-stearns -- I went through and fixed the rest of the mypy complaints locally and pushed that up here. Sorry about the back and forth on this and thanks again for contributing! I'll get this merged now that CI is happy.