Squashed 'manage_externals/' changes from fc5acdab..fde04e4d

fde04e4d Merge pull request ESCOMP#138 from billsacks/add_python38_tests
37e4c4a5 Do not update dictionary in-place in loop
7e8474bb Remove testing on mac os
7f41c563 Fix pylint issue
3065b0d6 Add travis-ci tests with python3.7 and python3.8
34fbf556 Add support for git sparse checkout
6c6ef9fe Fix pylint errors
6a659ad3 Added test for sparse checkout and updated documentation
14432439 Support for git sparsecheckout via read-tree.
a48558d8 Merge pull request ESCOMP#119 from gold2718/submodules
f72ffe7f Do not try git submodule update if no .gitmodules file (git bug)
804e0afb Fix a pylint error
45aef95e Addressed review concerns
7da50314 New capability to use git submodule information to checkout externals
1926530f Merge pull request ESCOMP#118 from mnlevy1981/svn_switch
b1b028d9 Updates after testing
9ea73e66 Add --svn-ignore-ancestry argument

git-subtree-dir: manage_externals
git-subtree-split: fde04e4d9a758b3aa277aa5fa44a59f5153f2958