Use / or \ in textplot if the function is increasing or decreasing

Before:

0.99845 |                                      ... ....
        |                                    ..        ..
        |                                   .            .
        |                                  .              .
        |                                 .                .
        |                                .                  .
        |                               .                    .
        | .                            .                      .
5.55112 | -.--------------------------.------------------------.-
        |   .                        .                          .
        |    .                      .
        |     .                    .
        |      .                  .
        |       .                .
        |        .              .
        |         .            .
        |          ..        ..
-0.9984 |            ........
          -10                    0                          10

After:

0.99845 |                                      ... ....
        |                                    ..        ..
        |                                   /            \
        |                                  /              \
        |                                 /                \
        |                                /                  \
        |                               /                    \
        | \                            /                      \
5.55112 | -\--------------------------/------------------------\-
        |   \                        /                          \
        |    \                      /
        |     \                    /
        |      \                  /
        |       \                /
        |        \              /
        |         \            /
        |          ..        ..
-0.9984 |            ........
          -10                    0                          10
Don't install isympy as isympy3 in Python 3
Fixed minor dummification issue.

Nothing too big, just looking at the namespaces used for eval rather than the
names of modules passed in.
Remove translation cruft.
Change the solve flag "minimal" to "particular"

Aside from the name being confusing, it was clobbering the minimal flag that
would otherwise be passed forward to checksol.
Added dummify flag to lambdify.

Replaced logic in lambdify which attempted to determine if dummification should
or should not happen, with a flag (default True) for dummification. The idea is
if someone wants to read the output and are already supplying 'sympy' as a
module, they shouldn't have a problem supplying dummify=False as an argument.
Fix some uses of solve(minimal=True) to solve(particular=True)
Corrections regarding PR 2428

Lambdify has no dummify option - it is default. Dummification only happens for
functions, not symbols.
Correct Python version requirements
Change formatting of the license years
Remove TODO (now it's redundant, see README.rst)
Fix spelling
Ignore sample.tex
Fix typo: relevent -> relevant
Merge pull request #2579 from skirpichev/misc

Misc small fixes
Merge pull request #2428 from gilbertgede/fix_lambdify

Fixed minor dummification issue.
Merge pull request #2418 from asmeurer/isympy3

Don't install isympy as isympy3 in Python 3
Merge pull request #2060 from asmeurer/textplot

Use / or \ in textplot if the function is increasing or decreasing
Merge pull request #2433 from jrioux/translation

Remove translation cruft.
Merge pull request #2437 from asmeurer/solve_minimal

Change the solve flag "minimal" to "particular"