Unpin pylint version and fix some lint warnings (#484)

* Fix unused import pandas lint warning on conversion.py
* Fix R1721: Unnecessary use of a comprehension
* Fix C0103: Variable name "v" doesn't conform
* Fix W1510: Using subprocess.run without explicitly set `check`
* Refresh pylintrc while keeping some old settings
* Fix C0415: Import outside toplevel
* Fix R1721: Unnecessary use of a comprehension
* Fix E0401: Unable to import
* Refactor text to reduce number of if-branches
* Simplify is_nonstr_iter into oneliner.
* Allow any import level for clib and clib.Session

Co-authored-by: Dongdong Tian <[email protected]>

Author: weiji14

.azure-pipelines.yml

@@ -21,7 +21,7 @@ jobs:
     vmImage: 'ubuntu-16.04'
 
   variables:
-    CONDA_INSTALL_EXTRA: "black flake8 pylint=2.2.2"
+    CONDA_INSTALL_EXTRA: "black flake8 pylint"
     PYTHON: '3.7'
 
   steps: