Randomgen

* Dropped support for Python 2.7
* Synchronized with the randomgen merges into NumPy 1.17 and their fixes coming in 1.18
* Added support for SeedSequence (and NumPy’s SeedSequence).
* The .generator method of the bit generators raise NotImplementedError
* The internal structures that is used in C have been renamed. The main rename is brng_t to bitgen_t
* Rename RandomGenerator to Generator.
* Rename randint() to integers().
* Rename random_integers() to integers().
* Rename random_sample() to random().
* Change jump which operated in-place to jumped() which returns a new BitGenerator.
* Rename Basic RNG to bit generator, which has been consistently applied across the docs and references
* Add the integer-based SIMD-based Fast Mersenne Twister (SFMT) generator SFMT.
* Add the 64-bit Mersenne Twister (MT64) generator MT64.
* Renamed Xoshiro256StarStar to Xoshiro256 and Xoshiro512StarStar to Xoshiro512

This release brings many breaking changes. Most of these have been implemented using DeprecationWarnings. This has been done to bring randomgen in-line with the API changes of the version going into NumPy.

* Dropped support for Python 2.7
* Synchronized with the randomgen merges into NumPy 1.17 and their fixes coming in 1.18
* Added support for SeedSequence (and NumPy’s SeedSequence).
* The .generator method of the bit generators raise NotImplementedError
* The internal structures that is used in C have been renamed. The main rename is brng_t to bitgen_t
* Rename RandomGenerator to Generator.
* Rename randint() to integers().
* Rename random_integers() to integers().
* Rename random_sample() to random().
* Change jump which operated in-place to jumped() which returns a new BitGenerator.
* Rename Basic RNG to bit generator, which has been consistently applied across the docs and references
* Add the integer-based SIMD-based Fast Mersenne Twister (SFMT) generator SFMT.
* Add the 64-bit Mersenne Twister (MT64) generator MT64.
* Renamed Xoshiro256StarStar to Xoshiro256 and Xoshiro512StarStar to Xoshiro512

This is a bug fix and performance fix:

- Changed the default jump step size to phi times the period of the generator for `PCG32` and `PCG64`.
- Improved the performance of `PCG64` on Windows.
- Improved performance of `DSFMT.jump` and `DSFMT.jumped`.
- Improves backward compatibility of `RandomState`

- Fixed bugs in multiple distributions that could result in `nan` values in rare circumstances (about 1 in 10E53)
- Added keyword ``closed`` to `randint` which changes sampling from the half-open interval ``[low, high)`` to the closed interval ``[low, high]``.
- Fixed a bug in `random_integers` that could lead to valid values being treated as invalid.

This is a feature release that is closely synchronized with the copy in NEP-19:

New features include:

- Add a fast path for broadcasting `RandomGenerator.randint` when using ``uint64`` or ``int64``.
- Refactor PCG64 so that it does not rely on Cython conditional compilation.
- Add `RandomGenerator.brng` to access the basic RNG.
- Allow multidimensional arrays in `RandomGenerator.choice`.
- Speed-up `RandomGenerator.choice` when not replacing. The gains can be very large (1000x or more) when the input array is large but the sample size is small.
- Add parameter checks in `RandomGenerator.multinomial`.
- Fix an edge-case bug in `RandomGenerator.zipf`.
- Allow 0 for sample in `RandomGenerator.hypergeometric`.
- Add broadcasting to `RandomGenerator.multinomial`.

This is a release that restores Python 2.7 compatibility.