Update sage.lfunctions.dokchitser.Dokchitser to use only one gp interpreter

[embray]

This updates the Dokchitser class to share a single gp interpreter instance for all instances of the Dokchitser class. Previously each instance spun up its own gp interpreter, and would keep it running until the instance is garbage-collected.

That situation was less than ideal in general, and even worse on platforms where file descriptors are limited (which could be any depending on the user's ulimits). See #22718

This provides a temporary workaround to something like #15809, which might be a preferable solution. But this solution was quick to hack together, works now, is needed for Cygwin, and will be easy enough to excise once a better solution is available. Please read the commit message for more implementation details.

(please set the correct component if not "elliptic curves"--it just seemed the most relevant out of what was available)

[embray]

A possible enhancement would be to allow an N-process pool, for faster parallel computation. But I don't know if that's really an important use-case here.

[git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

​2573de5

Modify the Dokchitser class to only require one gp interpreter which can

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​10fc86d	These variables aren't actually globals in this case and don't need to be indexed.
​44fe770	Unforunate (but working) hack needed in case any of the instantiation 'pre-code' references any of the indexed globals in the script template.

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​063041c

Use the appropriate built-in method for this

[embray]

Still finding new issues with this...

[git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

​906cc66	No need to unset CFLAGS for numpy
​2763fc9	Deprecation warning message should guide a user to import lcm from sage.arith.all
​ee626ea	22556: implement periodic points for rational maps
​cff3c89	22556: fixed doc typo
​308d4f2	22556: fix indeterminacy_locus
​4471c21	Updated SageMath version to 8.0.beta2
​a2a05ad	Modify the Dokchitser class to only require one gp interpreter which can
​17b67d4	Unforunate (but working) hack needed in case any of the instantiation 'pre-code' references any of the indexed globals in the script template.
​7e949ba	Use the appropriate built-in method for this
​492f57b	Initializing this attribute should be moved to the end of init_coeffs so that errors in initLdata are still raised as exceptions.

[git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

​dd6c49c	Modify the Dokchitser class to only require one gp interpreter which can
​a16bb52	Unforunate (but working) hack needed in case any of the instantiation 'pre-code' references any of the indexed globals in the script template.
​1aad963	Use the appropriate built-in method for this
​2cc45fa	Initializing this attribute should be moved to the end of init_coeffs so that errors in initLdata are still raised as exceptions.

[embray]

Okay, looks like I have this right now. I actually did have it working before but broke it with an unnecessary earlier commit that stemmed from misunderstanding how variable scoping works in pari.