PR summary

Python.h defines several visibility macros on my platform. If system headers are included before Python.h, they will define the visibility macros to default values, causing compilation errors due to the conflicting definitions. This follows python.org guidelines.

I moved the groups that looked like headers that included Python.h before all other include statements in the file.
These changes allow matplotlib to compile into a wheel on my platform (Cygwin).

PR checklist

• [N/A] "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• [N/A] Documentation complies with general and docstring guidelines