code cleanup and reformat

[jnozsc]

in this PR.
I did

• apply black to the code base, this is from PSF.
• apply flake8 to the code base, with only 3 ignore cases (E203, E501, W503):

• 1. E203 Whitespace before ':' there is a section talking about why we choose to ignore this
• 2. E501 Line too long. I try my best to reformat all long strings, but for some error message, I add #noqa: E501 to it
• 3. W503 Line break occurred before a binary operator . I have already fixed all cases, so flake8 should not complain about it. just keep this here, since black recommend

• remove/modify some python2 back compatibility codes, such as u"unicode_string" -> "unicode", since python2 support has been dropped
• leave docs/conf.py alone, since there are some #comment_me = True codes for the configuration
• add black and flake8 to travis

[lgtm-com]

This pull request introduces 3 alerts and fixes 3 when merging 037ee3c into 0f1ff4c - view on LGTM.com

new alerts:

• 3 for Explicit export is not defined

fixed alerts:

• 2 for Unused import
• 1 for 'import *' may pollute namespace

[lgtm-com]

This pull request fixes 3 alerts when merging e70f362 into 0f1ff4c - view on LGTM.com

fixed alerts:

• 2 for Unused import
• 1 for 'import *' may pollute namespace

[lgtm-com]

This pull request fixes 3 alerts when merging 30f0814 into 0f1ff4c - view on LGTM.com

fixed alerts:

• 2 for Unused import
• 1 for 'import *' may pollute namespace

[codecov]

Codecov Report

Merging #423 into master will decrease coverage by 0.16%.
The diff coverage is 74.56%.

@@            Coverage Diff             @@
##           master     #423      +/-   ##
==========================================
- Coverage   86.75%   86.59%   -0.17%     
==========================================
  Files          12       12              
  Lines        1533     1537       +4     
==========================================
+ Hits         1330     1331       +1     
- Misses        203      206       +3

Impacted Files	Coverage Δ
MySQLdb/_exceptions.py	100% <ø> (ø)	⬆️
MySQLdb/constants/ER.py	97.05% <0%> (-0.12%)	⬇️
MySQLdb/constants/CR.py	0% <0%> (ø)	⬆️
MySQLdb/times.py	100% <100%> (ø)	⬆️
MySQLdb/constants/__init__.py	100% <100%> (ø)	⬆️
MySQLdb/constants/CLIENT.py	100% <100%> (ø)	⬆️
MySQLdb/connections.py	81.25% <75%> (-0.57%)	⬇️
MySQLdb/converters.py	82.35% <77.77%> (ø)	⬆️
MySQLdb/cursors.py	80.93% <88%> (+0.31%)	⬆️
MySQLdb/__init__.py	81.81% <90%> (-1.52%)	⬇️
... and 1 more

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[lgtm-com]

This pull request fixes 3 alerts when merging ae4757d into 0f1ff4c - view on LGTM.com

fixed alerts:

• 2 for Unused import
• 1 for 'import *' may pollute namespace

[jnozsc]

for the Codacy test, I check the line which it complain on, it seems I format the line from single quotes to double quotes, and it just alert something as new. but in fact, the issue exists before I modify.

[lgtm-com]

This pull request fixes 3 alerts when merging eab8bcb into 0f1ff4c - view on LGTM.com

fixed alerts:

• 2 for Unused import
• 1 for 'import *' may pollute namespace

[methane]

Thank you.