Uproot

Fixes 205: `tree.iterate` on a TTree that has any zero-basket branches.

Merged 211, a feature allowing ROOT files to be read over authenticated HTTP (requiring the "requests" package) and 214, a bug-fix for matching branches to streamers under a TStreamerObject (see 213 for details).

Fixes 201. Large files have 64-bit seek points, which isn't reflected by the `TBranch.fBasketSeek` streamer; it depends on the preceding byte being `2`, rather than `1`.

Fixed 199: `uproot.iterate` and `uproot.numentries` skip files that don't have a TTree at the given `treepath`.

Now depends on awkward 0.7.0+ and uproot-methods 0.3.0+.

Strict dependency on the lz4 library was removed. It was difficult for some users to install and not needed for all ROOT files. If users encounter lz4-compressed or lzma-compressed ROOT files and they don't have these libraries, a message with copy-pasteable pip and conda installation commands are provided. This resolves the confusion raised in 186 and avoids 192 if possible.

The HTTPSource doesn't rely on `"content-length"` being correct if the total size is not larger than the chunk. This works around a bug where XRootD servers in HTTP mode were giving the wrong value. Issue 190.

Now `help(uproot)` returns a useful cheat-sheet. Issue 191.

All TTree access to Numpy is done through `awkward.util.numpy`, preparing for a test in which we swap Numpy for CuPy and read ROOT data directly into GPUs.

Depend on pkgconfig to try to resolve 192.