I used to think OpenGL was widely supported...

From PyGame’s front page:

“With many people having broken OpenGL setups, requiring OpenGL exclusively will cut into your user base significantly. Pygame uses either opengl, directx, windib, X11, linux frame buffer, and many other different backends… including an ASCII art backend! OpenGL is often broken on linux systems, and also on windows systems - which is why professional games use multiple backends.”

PyGame is often said to be a good choice for Python developers who want to support older hardware, but should I, as a newbie C++ game developer, be concerned that OpenGL isn’t as widespread as I thought before reading this?

OpenGL is very wide spread.
It’s available on many devices from Windows, Linux, Mac, Web browsers and xxxx million iPhones. Are you sure it’s not widespread?

MrMormon: you can also go with Direct3D, which is supported by everything from Microsoft and nowhere else…
If you want hardware accelerated 3D, OpenGL has the widest support (basically every device apart from Microsofts game consoles and phones - and i guess even those will support WebGL eventually).

Claiming OpenGL is not widely supported based on some linux users which insist on using only free drivers (which have a hard time catching up) is bogus.
Some OpenGL implementations have problems (even closed ones, i’m looking at you Intel, and also at you, Apple) but sadly there is no better alternative.