Add library version numbers to public API

[nulano]

Continuation of #4688.
Helps issues such as #4686, #3833.

Changes proposed in this pull request:

• Add access to library version numbers to public API in features module. Update tests to use this instead of private APIs.
• Use run-time version numbers where available; libjpeg apparently makes it unnecessarily hard to retrieve the version number.
• Add version numbers to pilinfo. Example from https://github.com/nulano/Pillow/runs/772850897?check_suite_focus=true:

--------------------------------------------------------------------
--- PIL CORE support ok, compiled for version 7.2.0.dev0
--- TKINTER support ok
--- FREETYPE2 support ok, loaded version 2.8.1
--- LITTLECMS2 support ok, loaded version 20.90
--- WEBP support ok, loaded version 0.6.1
--- WEBP Transparency support ok
--- WEBPMUX support ok
--- WEBP Animation support ok
--- JPEG support ok, compiled for version libjpeg-turbo 1.5.2
--- OPENJPEG (JPEG2000) support ok, loaded version 2.3.0
--- ZLIB (PNG/ZIP) support ok, loaded version 1.2.11
--- LIBTIFF support ok, loaded version 4.0.9
--- RAQM (Bidirectional Text) support ok, loaded version 0.7.0
--- LIBIMAGEQUANT (Quantization method) support ok, loaded version 2.12.6
--- XCB (X protocol) support ok
--------------------------------------------------------------------

I did not add a version number for XCB as I can't easily test it on Windows.

[hugovk]

Thanks!