CONTRIBUTING.md is missing mysql_config command being a developer dependency

[rjurney]

requirements-dev.txt requires mysqlclient which requires the OS dependency the mysql_config command. CONTRIBUTING.md does not let the new developer know this.

Expected results

pip install -r requirements-dev.txt

Expect: success!

Actual results

mysql_config not found!

How to reproduce the bug

1. Have a system without the mysql_config command on it
2. Run: pip install -r requirements-dev.txt
3. See error

Environment

• OS: Ubuntu 18.04 and OS X 10.14.4
• superset version: [release--0.31, b3aa5633]
• python version: 3.6.7, 3.6.8, 3.7.3

Checklist

Make sure these boxes are checked before submitting your issue - thank you!

• [*] I have checked the superset logs for python stacktraces and included it here as text if there are any.
• [*] I have reproduced the issue with at least the latest released version of superset.
• [*] I have checked the issue tracker for the same issue and I haven't found one similar.

Additional context

Add any other context about the problem here.

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[mistercrunch]

I found that pip install mysqlclient installs a newer version that works. I think we just need to bump the version in requirements-dev.txt

$ pip freeze | grep -i mysql
mysqlclient==1.4.2.post1

[rjurney]

@mistercrunch Updated so in master pull #7426

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[zhaoyongjie]

`mysql_config` missing, is a common environmental dependency issue. because mysqlclient need MySQLConnector/c. if we use pymysql instead of mysqlclient,these problems may be avoided. pymysql is a pure python mysql client.

…

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