minor cleanups of geographs

[martinfleis]

• removing get_neighbors and get_weights which are not needed anymore since we have __getitem__ implemented
• removing _id2i as it is unnecessary in sorted adjacency (and we don't need to keep anything in sync, yay!)
• updating indexing to eliminate pandas deprecation warnings
• fixing from_adjacency test to pass on windows

[codecov]

Codecov Report

Merging #559 (2e69a55) into geographs (8e2697c) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@             Coverage Diff             @@
##           geographs    #559     +/-   ##
===========================================
+ Coverage       81.1%   81.2%   +0.1%     
===========================================
  Files            127     127             
  Lines          14883   14856     -27     
===========================================
- Hits           12070   12062      -8     
+ Misses          2813    2794     -19     

Files Changed	Coverage Δ
libpysal/graph/tests/test_base.py	100.0% <ø> (ø)
libpysal/graph/base.py	95.7% <100.0%> (-0.2%)	⬇️
libpysal/graph/tests/test_builders.py	100.0% <100.0%> (ø)

... and 2 files with indirect coverage changes