Updated dev dependency requirements

[geleems]

• pytest
  • current installation version is 3.2.2, which conflicts with pytest-cov-2.7.1 requirement. pytest-cov-2.7.1 requires pytest>=3.6
  • pytest-4.6.5 is latest compatible version for both Python 2 and 3.
• nose
  • We are now using nose instead of pytest for auto test discovery in vscode python extension since there is an issue on pytest test discovery, which keeps throwing error message saying pytest is not installed while it is actually installed -- not resolved yet.
    • pytest is still being used for running unit tests in command prompt.
  • nose-1.3.7 is latest compatible version for both Python 2 and 3.
• pylint
  • Very useful tool for detecting wrong space characters and link breakers in vscode.
    • Message pops up suggesting use of pylint when opening mssql-cli workspace with vscode.
  • pylint-1.9.5 is latest compatible version for both Python 2 and 3.