Fix Latest pylint Errors

[sdimitro]

Commit 1: Fix latest pylint errors

Pylint started being more strict about the use of Python's
raise-from syntax explained here:
https://www.python.org/dev/peps/pep-3134/

The main idea is to provide a proper traceback whenever an
exception is caught and rethrown. Specifically in our case
though the use of raise-from shouldn't make any difference
for us because SDB is a REPL and prints the error messages
of our sdb-defined exceptions (which are basically faulty
exits of SDB commands).

Commit 2: Undo commit 9cd5599

The latest version of pylint has fixed the false positives
that we worked around on that commit.

• This will close Re-enable pylint not-callable warning for tests once pytest is updated. #235

Commit 3: update drgn type instantiation routines

drgn changed the way what you instantiate a type object, so we need to
update type_canonicalize().

• This commit was made by Matt as part of PR update drgn type instantiation routines #239

[codecov-commenter]

Codecov Report

Merging #240 into master will not change coverage.
The diff coverage is 88.23%.

@@           Coverage Diff           @@
##           master     #240   +/-   ##
=======================================
  Coverage   87.38%   87.38%           
=======================================
  Files          60       60           
  Lines        2473     2473           
=======================================
  Hits         2161     2161           
  Misses        312      312           

Impacted Files	Coverage Δ
sdb/commands/help.py	28.12% <0.00%> (ø)
sdb/commands/internal/table.py	93.33% <ø> (ø)
sdb/command.py	73.57% <100.00%> (ø)
sdb/commands/echo.py	100.00% <100.00%> (ø)
sdb/commands/linux/per_cpu.py	100.00% <100.00%> (ø)
sdb/commands/stacks.py	95.55% <100.00%> (ø)
sdb/pipeline.py	82.35% <100.00%> (ø)
sdb/target.py	97.61% <100.00%> (ø)

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