Add pypy support

[pganssle]

For now this only works with source distributions — I'd like to ship a PyPy wheel, but I can't figure out how to tag it so that it will only get picked up on PyPy.

I have an alternate version of this that actually allows the C extension to compile on PyPy, but on PyPy the C extension is way slower than the Python implementation (which for a lot of these benchmarks is actually faster than the C extension on CPython):

Running constructor in zone America/New_York
c_zoneinfo: mean: 490.79 ns ± 16.89 ns; min: 468.35 ns (k=5, N=1000000)
py_zoneinfo: mean: 49.54 ns ± 2.07 ns; min: 47.39 ns (k=5, N=10000000)
pytz: mean: 1.71 µs ± 70.73 ns; min: 1.66 µs (k=5, N=1000000)

Running constructor_strong_cache_miss in zone America/New_York
c_zoneinfo: mean: 28.57 µs ± 3.71 µs; min: 24.29 µs (k=5, N=10000)
py_zoneinfo: mean: 957.39 ns ± 40.07 ns; min: 913.89 ns (k=5, N=1000000)

Running from_utc in zone America/New_York
c_zoneinfo: mean: 4.93 µs ± 143.32 ns; min: 4.73 µs (k=5, N=100000)
py_zoneinfo: mean: 495.16 ns ± 41.14 ns; min: 452.01 ns (k=5, N=1000000)
pytz: mean: 627.67 ns ± 12.06 ns; min: 614.56 ns (k=5, N=1000000)

Running no_cache_constructor in zone America/New_York
c_zoneinfo: mean: 264.79 µs ± 36.98 µs; min: 229.38 µs (k=5, N=1000)
py_zoneinfo: mean: 106.52 µs ± 63.78 µs; min: 51.60 µs (k=5, N=1000)

Running to_utc in zone America/New_York
c_zoneinfo: mean: 3.44 µs ± 60.76 ns; min: 3.37 µs (k=5, N=100000)
py_zoneinfo: mean: 472.54 ns ± 39.36 ns; min: 432.28 ns (k=5, N=1000000)
pytz: mean: 332.76 ns ± 3.78 ns; min: 328.54 ns (k=5, N=1000000)

Running utcoffset in zone America/New_York
c_zoneinfo: mean: 3.45 µs ± 52.71 ns; min: 3.40 µs (k=5, N=100000)
py_zoneinfo: mean: 336.64 ns ± 11.60 ns; min: 325.69 ns (k=5, N=1000000)
pytz: mean: 213.31 ns ± 5.82 ns; min: 204.82 ns (k=5, N=10000000)

In the future, we'll probably want to be smarter about dropping the C extension tests entirely on PyPy, since this basically just runs all the same tests twice right now.