TST: fix tests of the Graph class

[martinfleis]

There are some flaky tests and I suppose that something does not play nice with pytest-xdist, so I disabled it now.

Some linting also happened.

[codecov]

Codecov Report

Merging #541 (2892e9a) into geographs (6d6bd0d) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@             Coverage Diff             @@
##           geographs    #541     +/-   ##
===========================================
+ Coverage       79.4%   79.6%   +0.1%     
===========================================
  Files            124     124             
  Lines          14229   14229             
===========================================
+ Hits           11303   11322     +19     
+ Misses          2926    2907     -19     

Files Changed	Coverage Δ
libpysal/graph/tests/test_base.py	100.0% <100.0%> (ø)
libpysal/graph/tests/test_contiguity.py	100.0% <100.0%> (ø)

... and 2 files with indirect coverage changes

[jGaboardi]

LOL at the bba0557 commit message.