[BUG] Geiser fails to start REPL on Emacs 26

[Ihor Radchenko]

Hi,

Our (Org mode) CI is now failing to run ob-scheme tests because Geiser
fails to run REPL:

https://builds.sr.ht/~bzg/job/1035753

Test test-ob-scheme/tables backtrace:
  (obsolete max-specpdl-size)()
  geiser-syntax--read-from-string("#<unspecified>")
  geiser-guile-update-warning-level()
  geiser-guile--startup(nil)
  apply(geiser-guile--startup nil)
  geiser-impl--call-method(repl-startup guile nil)
  geiser-repl--startup(guile nil)
  geiser-repl--start-repl(guile nil)
  geiser(guile)
  org-babel-scheme-get-repl(guile nil)

This happens because `geiser-syntax--read-from-string' tries to call
`with-suppressed-warnings', which is only available starting from Emacs
27.

Considering that geiser claims supporting Emacs >=25.1, this appears to
be a bug.

-- 
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
or support my work at <https://liberapay.com/yantar92>