Merge #14496: build: Pin to specific versions of Python packages we install from PyPI in Travis

ee0b7c4 build: Pin to specific versions of Python packages we install from PyPI in Travis (practicalswift)

Pull request description:

  Pin to specific versions of Python packages we install from PyPI in Travis.

  To avoid the possibility of surprise build failures when a new version of a PyPI dependency is released.

Tree-SHA512: 1c9abc094ad6b38992d0e7a5dd7f6c6c070a2eb81b052eb2397c1f1a489162cf8cedbd33db9bf97f32a81ed4dd0e986a76815c623a9b6c04b6ca6c5832d68211

Author: laanwj

.travis/lint_04_install.sh

@@ -7,4 +7,4 @@
 export LC_ALL=C
 
 travis_retry pip install codespell==1.13.0
-travis_retry pip install flake8
+travis_retry pip install flake8==3.5.0