[Python][CI] Upgrade to latest flake8 3.7.5 in travis_lint.sh

[asfimport]

We are currently pinned to flake8 3.5. The latest one turns up some additional warnings

C:\Users\wesmc\code\arrow (master -> origin)
(arrow-dev) λ flake8 dev\
dev\merge_arrow_pr.py:60:1: E302 expected 2 blank lines, found 1
dev\merge_arrow_pr.py:64:1: E302 expected 2 blank lines, found 1
dev\merge_arrow_pr.py:99:35: W605 invalid escape sequence '\['
dev\merge_arrow_pr.py:99:39: W605 invalid escape sequence '\]'
dev\merge_arrow_pr.py:99:43: W605 invalid escape sequence '\]'
dev\merge_arrow_pr.py:146:46: W605 invalid escape sequence '\d'
dev\merge_arrow_pr.py:454:5: F841 local variable 'e' is assigned to but never used
dev\release\changelog.py:80:25: W605 invalid escape sequence '\_'
dev\release\changelog.py:81:24: W605 invalid escape sequence '\`'
dev\release\changelog.py:82:24: W605 invalid escape sequence '\*'

C:\Users\wesmc\code\arrow (master -> origin)
(arrow-dev) λ flake8 python\
python\pyarrow\filesystem.py:388:16: F632 use ==/!= to compare str, bytes, and int literals
python\pyarrow\filesystem.py:392:18: F632 use ==/!= to compare str, bytes, and int literals

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #3616

_{Note: This issue was originally created as ARROW-4499. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 3616
#3616