Update python_requires to match supported python versions

[gopackgo90]

In https://github.com/PyMySQL/mysqlclient/commit/684dcbf0657f18c1ba12fe21732323c890ff20ab and https://github.com/PyMySQL/mysqlclient/commit/dac24e7b05b83eb1511e25e3cd8f8c20b7fbe112 it looks like Python 3.6 support has been dropped. This change updates python_requires to match so tools like pip know not to install the next version in a Python 3.6 environment.

[codecov[bot]]

Codecov Report

Merging #543 (8686961) into main (dac24e7) will not change coverage. The diff coverage is n/a.

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  Coverage   86.00%   86.00%           
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  Files           6        6           
  Lines         550      550           
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  Hits          473      473           
  Misses         77       77           

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[methane]

Those commit drop only official support. It doesn't mean current mysqlclient doesn't work with Python 3.6. I don't want to block users try using recent mysqlclient with Python 3.6. Is it easy to ignore python_requires?

[gopackgo90]

You're already preventing users from installing the latest version of this library on Python 3.5. What makes Python 3.6 so special that despite dropping official support for/testing on it you still want to allow users to try installing it in their Python 3.6 environments?

If you want to allow users to keep installing the latest versions on Python 3.6, why drop official support for it at all? The only change looked to be that you stopped advertising support for it and stopped automated testing against it.

If at some point a user would have an issue installing/using this library on Python 3.6, would you work on resolving that issue? If not, then why let users shoot themselves in the foot? If so, why drop "official" support for Python 3.6 if you would support resolving issues related to it anyways?

And no, I don't think python_requires checks are easy to bypass.

[methane]

I don't remember correctly, but there were several reasons I want to drop Python 3.5.

For example, this commit drop Python 3.5 support. So I am sure that current code don't work with Python 3.5. https://github.com/PyMySQL/mysqlclient/commit/5e8eeac47f511ca63d6b40dcc3a47190d96a0c0d

On the other hand, I don't have explicit reason to block installing for Python 3.6. I drop official support for Python 3.6 only because Python 3.6 reached EOL.